class LastUserField(models.ForeignKey):
    """ 
    A field that keeps the last user that saved an instance
    of a model. None will be the value for AnonymousUser.
    """

def __init__(self, **kwargs):
    kwargs.pop('null', None)
kwargs.pop('to', None)
super(LastUserField, self).__init__(User, null=True, **kwargs)

def contribute_to_class(self, cls, name):
    super(LastUserField, self).contribute_to_class(cls, name)
registry = registration.FieldRegistry(self.__class__)
registry.add_field(cls, self)
django-south with django-audit-log