Index: django/db/models/fields/__init__.py
===================================================================
--- django/db/models/fields/__init__.py	(revision 2628)
+++ django/db/models/fields/__init__.py	(working copy)
@@ -423,7 +423,7 @@
         return Field.get_db_prep_lookup(self, lookup_type, value)
 
     def pre_save(self, value, add):
-        if self.auto_now or (self.auto_now_add and add):
+        if self.auto_now or (self.auto_now_add and add and not value):
             return datetime.datetime.now()
         return value