diff --git a/IPython/config/configurable.py b/IPython/config/configurable.py
index 1d63107..a0dae9f 100755
--- a/IPython/config/configurable.py
+++ b/IPython/config/configurable.py
@@ -232,3 +232,9 @@ class SingletonConfigurable(Configurable):
                 'Multiple incompatible subclass instances of '
                 '%s are being created.' % cls.__name__
             )
+
+    @classmethod
+    def initialized(cls):
+        """Has an instance been created?"""
+        return hasattr(cls, "_instance") and cls._instance is not None
+
diff --git a/IPython/config/tests/test_configurable.py b/IPython/config/tests/test_configurable.py
index 2a5a4ac..7fc63ff 100644
--- a/IPython/config/tests/test_configurable.py
+++ b/IPython/config/tests/test_configurable.py
@@ -152,17 +152,21 @@ class TestSingletonConfigurable(TestCase):
     def test_instance(self):
         from IPython.config.configurable import SingletonConfigurable
         class Foo(SingletonConfigurable): pass
+        self.assertEquals(Foo.initialized(), False)
         foo = Foo.instance()
+        self.assertEquals(Foo.initialized(), True)
         self.assertEquals(foo, Foo.instance())
         self.assertEquals(SingletonConfigurable._instance, None)
 
     def test_inheritance(self):
-
         class Bar(SingletonConfigurable): pass
         class Bam(Bar): pass
+        self.assertEquals(Bar.initialized(), False)
+        self.assertEquals(Bam.initialized(), False)
         bam = Bam.instance()
         bam == Bar.instance()
+        self.assertEquals(Bar.initialized(), True)
+        self.assertEquals(Bam.initialized(), True)
         self.assertEquals(bam, Bam._instance)
         self.assertEquals(bam, Bar._instance)
         self.assertEquals(SingletonConfigurable._instance, None)
-        
\ No newline at end of file
diff --git a/IPython/core/interactiveshell.py b/IPython/core/interactiveshell.py
index 3ba7de4..c0e75f7 100644
--- a/IPython/core/interactiveshell.py
+++ b/IPython/core/interactiveshell.py
@@ -31,7 +31,7 @@ import tempfile
 import types
 from contextlib import nested
 
-from IPython.config.configurable import Configurable
+from IPython.config.configurable import SingletonConfigurable
 from IPython.core import debugger, oinspect
 from IPython.core import history as ipcorehist
 from IPython.core import page
@@ -132,9 +132,7 @@ class SeparateStr(Str):
         value = value.replace('\\n','\n')
         return super(SeparateStr, self).validate(obj, value)
 
-class MultipleInstanceError(Exception):
-    pass
-    
+
 class ReadlineNoRecord(object):
     """Context manager to execute some code, then reload readline history
     so that interactive input to the code doesn't appear when pressing up."""
@@ -194,7 +192,7 @@ called (even if no arguments are present). The default is '1'.
 # Main IPython class
 #-----------------------------------------------------------------------------
 
-class InteractiveShell(Configurable, Magic):
+class InteractiveShell(SingletonConfigurable, Magic):
     """An enhanced, interactive shell for Python."""
 
     _instance = None
@@ -427,31 +425,6 @@ class InteractiveShell(Configurable, Magic):
         self.hooks.late_startup_hook()
         atexit.register(self.atexit_operations)
 
-    @classmethod
-    def instance(cls, *args, **kwargs):
-        """Returns a global InteractiveShell instance."""
-        if cls._instance is None:
-            inst = cls(*args, **kwargs)
-            # Now make sure that the instance will also be returned by
-            # the subclasses instance attribute.
-            for subclass in cls.mro():
-                if issubclass(cls, subclass) and \
-                       issubclass(subclass, InteractiveShell):
-                    subclass._instance = inst
-                else:
-                    break
-        if isinstance(cls._instance, cls):
-            return cls._instance
-        else:
-            raise MultipleInstanceError(
-                'Multiple incompatible subclass instances of '
-                'InteractiveShell are being created.'
-            )
-
-    @classmethod
-    def initialized(cls):
-        return hasattr(cls, "_instance")
-
     def get_ipython(self):
         """Return the currently running IPython instance."""
         return self