Index: django/contrib/sessions/middleware.py
===================================================================
--- django/contrib/sessions/middleware.py	(revision 5559)
+++ django/contrib/sessions/middleware.py	(working copy)
@@ -37,6 +37,7 @@
         return self._session.get(key, default)
 
     def pop(self, key, *args):
+        self.modified = self.modified or key in self._session 
         return self._session.pop(key, *args)
 
     def set_test_cookie(self):
Index: django/contrib/sessions/tests.py
===================================================================
--- django/contrib/sessions/tests.py	(revision 5559)
+++ django/contrib/sessions/tests.py	(working copy)
@@ -5,8 +5,24 @@
 >>> s._session_cache = {}
 >>> s._session_cache['some key'] = 'exists'
 
+>>> s.accessed
+False
+>>> s.modified
+False
+
+>>> s.pop('non existant key', 'does not exist')
+'does not exist'
+>>> s.accessed
+True
+>>> s.modified
+False
+
 >>> s.pop('some key')
 'exists'
+>>> s.accessed
+True
+>>> s.modified
+True
 
 >>> s.pop('some key', 'does not exist')
 'does not exist'