Just ran into a South migration issue.  Basically we had a field that looked like this (simplified):

class MyModel( models.Model ):
    hostname = models.CharField(
        default = random_unique_hostname,
    )

where random_unique_hostname() was written like so (again, simplified):

def random_unique_hostname():
    test = generate_random()
    try:
        MyModel.objects.get( hostname = test )
    except MyModel.DoesNotExist, err:
        return test
    else:
        return random_unique_hostname()

the actual code is significantly more complex, but that illustrates the problem.  When you try to generate a South migration to add some *other* field to this model (i.e. a few months after you've added this default), you'll see the migration creation fail because South is calling your default (ouch) with a database that doesn't have the new field (the pre-migration DB).

The solution is to use a minimal query, rather than calling .get, use .filter().count() or use .filter().values_list('id') or any other query that doesn't map to the whole object being loaded, just to check for unique-ness.