diff -r b758351d191f -r cc9b7e14412b web/lib/django/contrib/admin/views/main.py
--- a/web/lib/django/contrib/admin/views/main.py	Wed May 19 17:43:59 2010 +0200
+++ b/web/lib/django/contrib/admin/views/main.py	Tue May 25 02:43:45 2010 +0200
@@ -9,11 +9,6 @@
 from django.utils.http import urlencode
 import operator
 
-try:
-    set
-except NameError:
-    from sets import Set as set   # Python 2.3 fallback
-
 # The system will display a "Show all" link on the change list only if the
 # total result count is less than or equal to this setting.
 MAX_SHOW_ALL_ALLOWED = 200
@@ -185,6 +180,13 @@
             if key.endswith('__in'):
                 lookup_params[key] = value.split(',')
 
+            # if key ends with __isnull, special case '' and false
+            if key.endswith('__isnull'):
+                if value.lower() in ('', 'false'):
+                    lookup_params[key] = False
+                else:
+                    lookup_params[key] = True
+
         # Apply lookup parameters from the query string.
         try:
             qs = qs.filter(**lookup_params)