add disabled state to undo-delete-cell when unavailable.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -546,6 +546,7 b' var IPython = (function (IPython) {'
         var i = this.index_or_selected(index);
         var cell = this.get_selected_cell();
         this.undelete_backup = cell.toJSON();
+        $('#undelete_cell').removeClass('ui-state-disabled');
         if (this.is_valid_cell_index(i)) {
             var ce = this.get_cell_element(i);
             ce.remove();
@@ -866,6 +867,7 b' var IPython = (function (IPython) {'
             this.undelete_backup = null;
             this.undelete_index = null;
         }
+        $('#undelete_cell').addClass('ui-state-disabled');
     }
 
     // Split/merge

IPython/frontend/html/notebook/templates/notebook.html

@@ -78,7 +78,7 b' class="notebook_app"'
                 <li id="paste_cell_below" class="ui-state-disabled"><a href="#">Paste Cell Below</a></li>
                 <li id="paste_cell_replace" class="ui-state-disabled"><a href="#">Paste Cell &amp; Replace</a></li>
                 <li id="delete_cell"><a href="#">Delete Cell</a></li>
-                <li id="undelete_cell"><a href="#">Undo Delete Cell</a></li>
+                <li id="undelete_cell" class="ui-state-disabled"><a href="#">Undo Delete Cell</a></li>
                 <hr/>
                 <li id="split_cell"><a href="#">Split Cell</a></li>
                 <li id="merge_cell_above"><a href="#">Merge Cell Above</a></li>