The decorator actually raises an interesting issue. Are permissions and security part of the core application logic, or are they wrappers.

The mixin approach is nice and OO, but it's potentially tying two different layers of functionality together. Whereas the decorator is closer to aspect-oriented programming which attempts to isolate core business logic from such modifiers.

I'm actually quite interested in the challenge of applying design patterns for large systems (inversion-of-control, AOP etc) to Django, which has some stubborn behaviours that get in the way.