diff --git a/mercurial/subrepo.py b/mercurial/subrepo.py
--- a/mercurial/subrepo.py
+++ b/mercurial/subrepo.py
@@ -163,6 +163,17 @@ def submerge(repo, wctx, mctx, actx, ove
     # record merged .hgsubstate
     writestate(repo, sm)
 
+def _updateprompt(ui, sub, dirty, local, remote):
+    if dirty:
+        msg = (_(' subrepository sources for %s differ\n'
+                 'use (l)ocal source (%s) or (r)emote source (%s)?\n')
+               % (subrelpath(sub), local, remote))
+    else:
+        msg = (_(' subrepository sources for %s differ (in checked out version)\n'
+                 'use (l)ocal source (%s) or (r)emote source (%s)?\n')
+               % (subrelpath(sub), local, remote))
+    return ui.promptchoice(msg, (_('&Local'), _('&Remote')), 0)
+
 def reporelpath(repo):
     """return path to this (sub)repo as seen from outermost repo"""
     parent = repo
@@ -442,14 +453,26 @@ class hgsubrepo(abstractsubrepo):
         cur = self._repo['.']
         dst = self._repo[state[1]]
         anc = dst.ancestor(cur)
-        if anc == cur:
-            self._repo.ui.debug("updating subrepo %s\n" % subrelpath(self))
-            hg.update(self._repo, state[1])
-        elif anc == dst:
-            self._repo.ui.debug("skipping subrepo %s\n" % subrelpath(self))
+
+        def mergefunc():
+            if anc == cur:
+                self._repo.ui.debug("updating subrepo %s\n" % subrelpath(self))
+                hg.update(self._repo, state[1])
+            elif anc == dst:
+                self._repo.ui.debug("skipping subrepo %s\n" % subrelpath(self))
+            else:
+                self._repo.ui.debug("merging subrepo %s\n" % subrelpath(self))
+                hg.merge(self._repo, state[1], remind=False)
+
+        wctx = self._repo[None]
+        if self.dirty():
+            if anc != dst:
+                if _updateprompt(self._repo.ui, self, wctx.dirty(), cur, dst):
+                    mergefunc()
+            else:
+                mergefunc()
         else:
-            self._repo.ui.debug("merging subrepo %s\n" % subrelpath(self))
-            hg.merge(self._repo, state[1], remind=False)
+            mergefunc()
 
     def push(self, force):
         # push subrepos depth-first for coherent ordering
@@ -608,10 +631,12 @@ class svnsubrepo(abstractsubrepo):
         self._ui.status(status)
 
     def merge(self, state):
-        old = int(self._state[1])
-        new = int(state[1])
-        if new > old:
-            self.get(state)
+        old = self._state[1]
+        new = state[1]
+        if new != self._wcrev():
+            dirty = old == self._wcrev() or self._wcchanged()[0]
+            if _updateprompt(self._ui, self, dirty, self._wcrev(), new):
+                self.get(state, False)
 
     def push(self, force):
         # push is a no-op for SVN
@@ -850,10 +875,21 @@ class gitsubrepo(abstractsubrepo):
         source, revision, kind = state
         self._fetch(source, revision)
         base = self._gitcommand(['merge-base', revision, self._state[1]])
-        if base == revision:
-            self.get(state) # fast forward merge
-        elif base != self._state[1]:
-            self._gitcommand(['merge', '--no-commit', revision])
+        out, code = self._gitdir(['diff-index', '--quiet', 'HEAD'])
+
+        def mergefunc():
+            if base == revision:
+                self.get(state) # fast forward merge
+            elif base != self._state[1]:
+                self._gitcommand(['merge', '--no-commit', revision])
+
+        if self.dirty():
+            if self._gitstate() != revision:
+                dirty = self._gitstate() == self._state[1] or code != 0
+                if _updateprompt(self._ui, self, dirty, self._state[1], revision):
+                    mergefunc()
+        else:
+            mergefunc()
 
     def push(self, force):
         # if a branch in origin contains the revision, nothing to do
diff --git a/tests/test-subrepo-git.t b/tests/test-subrepo-git.t
--- a/tests/test-subrepo-git.t
+++ b/tests/test-subrepo-git.t
@@ -329,3 +329,117 @@ Check hg update --clean
   f1
   f2
   g
+
+Sticky subrepositories, no changes
+  $ cd $TESTTMP/ta
+  $ hg id -n
+  7
+  $ cd s
+  $ git rev-parse HEAD
+  32a343883b74769118bb1d3b4b1fbf9156f4dddc
+  $ cd ..
+  $ hg update 1 > /dev/null 2>&1
+  $ hg id -n
+  1
+  $ cd s
+  $ git rev-parse HEAD
+  da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7
+  $ cd ..
+
+Sticky subrepositorys, file changes
+  $ touch s/f1
+  $ cd s
+  $ git add f1
+  $ cd ..
+  $ hg id -n
+  1
+  $ cd s
+  $ git rev-parse HEAD
+  da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7
+  $ cd ..
+  $ hg update 4
+   subrepository sources for s differ
+  use (l)ocal source (da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7) or (r)emote source (aa84837ccfbdfedcdcdeeedc309d73e6eb069edc)?
+   l
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  4+
+  $ cd s
+  $ git rev-parse HEAD
+  da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7
+  $ cd ..
+  $ hg update --clean tip > /dev/null 2>&1 
+
+Sticky subrepository, revision updates
+  $ hg id -n
+  7
+  $ cd s
+  $ git rev-parse HEAD
+  32a343883b74769118bb1d3b4b1fbf9156f4dddc
+  $ cd ..
+  $ cd s
+  $ git checkout aa84837ccfbdfedcdcdeeedc309d73e6eb069edc
+  Previous HEAD position was 32a3438... fff
+  HEAD is now at aa84837... f
+  $ cd ..
+  $ hg update 1
+   subrepository sources for s differ (in checked out version)
+  use (l)ocal source (32a343883b74769118bb1d3b4b1fbf9156f4dddc) or (r)emote source (da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7)?
+   l
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  1+
+  $ cd s
+  $ git rev-parse HEAD
+  aa84837ccfbdfedcdcdeeedc309d73e6eb069edc
+  $ cd ..
+
+Sticky subrepository, file changes and revision updates
+  $ touch s/f1
+  $ cd s
+  $ git add f1
+  $ git rev-parse HEAD
+  aa84837ccfbdfedcdcdeeedc309d73e6eb069edc
+  $ cd ..
+  $ hg id -n
+  1+
+  $ hg update 7
+   subrepository sources for s differ
+  use (l)ocal source (32a343883b74769118bb1d3b4b1fbf9156f4dddc) or (r)emote source (32a343883b74769118bb1d3b4b1fbf9156f4dddc)?
+   l
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  7
+  $ cd s
+  $ git rev-parse HEAD
+  aa84837ccfbdfedcdcdeeedc309d73e6eb069edc
+  $ cd ..
+
+Sticky repository, update --clean
+  $ hg update --clean tip
+  Previous HEAD position was aa84837... f
+  HEAD is now at 32a3438... fff
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  7
+  $ cd s
+  $ git rev-parse HEAD
+  32a343883b74769118bb1d3b4b1fbf9156f4dddc
+  $ cd ..
+
+Test subrepo already at intended revision:
+  $ cd s
+  $ git checkout 32a343883b74769118bb1d3b4b1fbf9156f4dddc
+  HEAD is now at 32a3438... fff
+  $ cd ..
+  $ hg update 1
+  Previous HEAD position was 32a3438... fff
+  HEAD is now at da5f5b1... g
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  1
+  $ cd s
+  $ git rev-parse HEAD
+  da5f5b1d8ffcf62fb8327bcd3c89a4367a6018e7
+  $ cd ..
+
diff --git a/tests/test-subrepo-svn.t b/tests/test-subrepo-svn.t
--- a/tests/test-subrepo-svn.t
+++ b/tests/test-subrepo-svn.t
@@ -296,3 +296,145 @@ Check hg update --clean
   ? *    f2 (glob)
   
   Performing status on external item at 'externals'
+
+Sticky subrepositories, no changes
+  $ cd $TESTTMP/sub/t
+  $ hg id -n
+  2
+  $ cd s
+  $ svnversion
+  3
+  $ cd ..
+  $ hg update 1
+  U    $TESTTMP/sub/t/s/alpha
+  
+  Fetching external item into '$TESTTMP/sub/t/s/externals'
+  Checked out external at revision 1.
+  
+  Checked out revision 2.
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  1
+  $ cd s
+  $ svnversion
+  2
+  $ cd ..
+
+Sticky subrepositorys, file changes
+  $ touch s/f1
+  $ cd s
+  $ svn add f1
+  A         f1
+  $ cd ..
+  $ hg id -n
+  1
+  $ cd s
+  $ svnversion
+  2M
+  $ cd ..
+  $ hg update tip
+   subrepository sources for s differ
+  use (l)ocal source (2) or (r)emote source (3)?
+   l
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  2+
+  $ cd s
+  $ svnversion
+  2M
+  $ cd ..
+  $ hg update --clean tip
+  U    $TESTTMP/sub/t/s/alpha
+  
+  Fetching external item into '$TESTTMP/sub/t/s/externals'
+  Checked out external at revision 1.
+  
+  Checked out revision 3.
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Sticky subrepository, revision updates
+  $ hg id -n
+  2
+  $ cd s
+  $ svnversion
+  3
+  $ cd ..
+  $ cd s
+  $ svn update -r 1
+  U    alpha
+   U   .
+  
+  Fetching external item into 'externals'
+  Updated external to revision 1.
+  
+  Updated to revision 1.
+  $ cd ..
+  $ hg update 1
+   subrepository sources for s differ (in checked out version)
+  use (l)ocal source (1) or (r)emote source (2)?
+   l
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  1+
+  $ cd s
+  $ svnversion
+  1
+  $ cd ..
+
+Sticky subrepository, file changes and revision updates
+  $ touch s/f1
+  $ cd s
+  $ svn add f1
+  A         f1
+  $ svnversion
+  1M
+  $ cd ..
+  $ hg id -n
+  1+
+  $ hg update tip
+   subrepository sources for s differ
+  use (l)ocal source (1) or (r)emote source (3)?
+   l
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  2
+  $ cd s
+  $ svnversion
+  1M
+  $ cd ..
+
+Sticky repository, update --clean
+  $ hg update --clean tip
+  U    $TESTTMP/sub/t/s/alpha
+   U   $TESTTMP/sub/t/s
+  
+  Fetching external item into '$TESTTMP/sub/t/s/externals'
+  Checked out external at revision 1.
+  
+  Checked out revision 3.
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  2
+  $ cd s
+  $ svnversion
+  3
+  $ cd ..
+
+Test subrepo already at intended revision:
+  $ cd s
+  $ svn update -r 2
+  U    alpha
+  
+  Fetching external item into 'externals'
+  Updated external to revision 1.
+  
+  Updated to revision 2.
+  $ cd ..
+  $ hg update 1
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  1+
+  $ cd s
+  $ svnversion
+  2
+  $ cd ..
diff --git a/tests/test-subrepo.t b/tests/test-subrepo.t
--- a/tests/test-subrepo.t
+++ b/tests/test-subrepo.t
@@ -706,3 +706,125 @@ Check hg update --clean
   $ hg status -S
   ? s/b
   ? s/c
+
+Sticky subrepositories, no changes
+  $ cd $TESTTMP/sub/t
+  $ hg id
+  925c17564ef8 tip
+  $ hg -R s id
+  12a213df6fa9 tip
+  $ hg -R t id  
+  52c0adc0515a tip
+  $ hg update 11
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  365661e5936a
+  $ hg -R s id
+  fc627a69481f
+  $ hg -R t id  
+  e95bcfa18a35
+
+Sticky subrepositorys, file changes
+  $ touch s/f1
+  $ touch t/f1
+  $ hg add -S s/f1
+  $ hg add -S t/f1
+  $ hg id
+  365661e5936a
+  $ hg -R s id
+  fc627a69481f+
+  $ hg -R t id  
+  e95bcfa18a35+
+  $ hg update tip
+   subrepository sources for s differ
+  use (l)ocal source (fc627a69481f) or (r)emote source (12a213df6fa9)?
+   l
+   subrepository sources for t differ
+  use (l)ocal source (e95bcfa18a35) or (r)emote source (52c0adc0515a)?
+   l
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  925c17564ef8+ tip
+  $ hg -R s id
+  fc627a69481f+
+  $ hg -R t id  
+  e95bcfa18a35+
+  $ hg update --clean tip
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+Sticky subrepository, revision updates
+  $ hg id
+  925c17564ef8 tip
+  $ hg -R s id
+  12a213df6fa9 tip
+  $ hg -R t id  
+  52c0adc0515a tip
+  $ cd s
+  $ hg update -r -2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd ../t
+  $ hg update -r 2
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd ..
+  $ hg update 10
+   subrepository sources for t differ (in checked out version)
+  use (l)ocal source (7af322bc1198) or (r)emote source (20a0db6fbf6c)?
+   l
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  e45c8b14af55+
+  $ hg -R s id
+  1c833a7a9e3a
+  $ hg -R t id  
+  7af322bc1198
+
+Sticky subrepository, file changes and revision updates
+  $ touch s/f1
+  $ touch t/f1
+  $ hg add -S s/f1
+  $ hg add -S t/f1
+  $ hg id
+  e45c8b14af55+
+  $ hg -R s id
+  1c833a7a9e3a+
+  $ hg -R t id  
+  7af322bc1198+
+  $ hg update tip
+   subrepository sources for s differ
+  use (l)ocal source (1c833a7a9e3a) or (r)emote source (12a213df6fa9)?
+   l
+   subrepository sources for t differ
+  use (l)ocal source (7af322bc1198) or (r)emote source (52c0adc0515a)?
+   l
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  925c17564ef8 tip
+  $ hg -R s id
+  1c833a7a9e3a+
+  $ hg -R t id  
+  7af322bc1198+
+
+Sticky repository, update --clean
+  $ hg update --clean tip
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  925c17564ef8 tip
+  $ hg -R s id
+  12a213df6fa9 tip
+  $ hg -R t id  
+  52c0adc0515a tip
+
+Test subrepo already at intended revision:
+  $ cd s
+  $ hg update fc627a69481f
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cd ..
+  $ hg update 11
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id -n
+  11+
+  $ hg -R s id
+  fc627a69481f
+  $ hg -R t id 
+  e95bcfa18a35