I've added some minor fixes to make it easier to use django-registration with custom User models.
Modified create_inactive_user to use kwargs, so that create_user implementations can more easily gobble up/ignore the username or email arg.
Checked for get_username(), as per the new specification. A further improvement here could be to allow users to define a get_hash_values(), or something similar, to allow developers to specify their own generation method.
My vim configuration strips whitespace, so I apologise for the bigger than expected diff.