Index: django/db/models/base.py
===================================================================
--- django/db/models/base.py	(revision 4910)
+++ django/db/models/base.py	(working copy)
@@ -88,6 +88,9 @@
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    def __hash__(self):
+        return hash(str(self._get_pk_val()))
+
     def __init__(self, *args, **kwargs):
         dispatcher.send(signal=signals.pre_init, sender=self.__class__, args=args, kwargs=kwargs)
         for f in self._meta.fields: