The short answer is that nobody knows...
This is a whole different ballgame this time around, so we can't look at either of the previous iPhone releases for hints.
Personally, I think AT&T would be shooting themselves in the foot if they didn't provide current 3G and original iPhone owners with automatic upgrade eligibility because they will move a lot more volume if they do. Apple has almost a cult following and AT&T would be pretty silly to ignore that.
On the other hand, AT&T does have a lot of money invested in the subsidizing of the 3G iPhones that they have sold. I can see them wanting to recoup as much as possible and therefore leaving the usual upgrade terms intact. But, then again, I suspect that their pricing structure is such that they actually break even a lot faster than they let on, so who knows.
It will be interesting to see, though, that's for sure.