I'm with nixgeek on this one, however it must be said that nobody but Apple knows what will be released and/or supported in the future, and anyone that claims to know definitively is either speculating or lying.
The only way to tell for absolute sure is to wait and see.
Like nixgeek said, I speculate it will be AT&T and the cost will be identical to what it costs today for monthly service with the 3GS.