I submitted a patch to Django Ticket #399 (request for a bigint field type). It still needs testing but works at a quick glance on mysql. Here’s a shot of them in action from the admin interface (the input is just too small and just too big respectively):
Update: BigIntegerField works perfectly on PostgreSQL but because it doesn’t have an unsigned integer type (that I can find), PostitiveBigIntegerField isn’t going to make it all the way up to 18,446,744,073,709,551,615 without using an arbitrary precision NUMERIC or mapping zero to -9,223,372,036,854,775,808. Both solutions are messy and it would be a shame to have the mysql and postgres backends behave so differently. As an aside, it looks like this is already the case with mysql’s IntegerFields being UNSIGNED while Postgres just checks to make sure that the integers are positive before inserting.
The best solution would probably be to employ backend-specific range checking for these monsterous numbers. That way you won’t end up out of range in PostgreSQL but you’re also not penalizing MySQL for being able to count to 18 bajillion. At this point it would be safe to drop in BigIntegerField as is (as soon as I check it out on sqlite), but PostitiveBigIntegerField still needs some pondering.