You can also subclass auth.User, which allows you to maintain compatibility with 3rd party apps.
This is how I've gotten around the user name length limitation. I have a longer field in the subclass, and implement a backend to check that. But because it's a subclass, foreign-key relations still work.
What if you want to get rid of the username field altogether?
For example, my typical use case: unique email address for logins, no usernames:
I suppose you could subclass User with a longer username, and sync the username with the email address using a signal, for example.
Still, it's a chunk of workaround I end up doing one way or another with each new project. Hopefully soon (based on discussions on django-developers group) we'll see some kind of pluggable User model.
This is how I've gotten around the user name length limitation. I have a longer field in the subclass, and implement a backend to check that. But because it's a subclass, foreign-key relations still work.