When you use the User.get_profile() method to fetch a profile object, it doesn't pre-populate profile's user attribute. This results in unnecessary DB queries when you go from User -> UserProfile and then later go from UserProfile -> User. This is actually a fairly common use case because people tend to attach methods that "extend" the user object in UserProfile, since they can't directly extend User.
For example, if you create a full_name() method on UserProfile that returns '%s %s' % (self.user.first_name, self.user.last_name), then in your templates, you do {{ User.get_profile.full_name }} , you end up doing two DB queries when only one in necessary.
The solution seems simple enough since the profile's User object must be called user. In django/contrib/auth/models.py a one liner after fetching the profile should do the trick:
self._profile_cache = model._default_manager.get(user__id__exact=self.id)
+ self._profile_cache.user = self
(The reason this isn't a real patch is because mmalone was working off a non-trunk checkout for his commercial stuff. So I asked him just to file the right line and we can turn it into a trivial patch later. Just noting so nobody wonders why he's being a bad patch writer.)