If they drop the 3GS, and I think they will, my hope is Apple and AT&T at least provide some sort of modified upgrade discount policy for 3GS owners. By the time the iPhone 5 (or 4S) comes out, in September or October presumably, there's going to be an awful lot of people who bought 3GSes in the first half of this year that get ticked off because a phone that is still being heavily promoted (saw two of those 3GS commercials yesterday) can't run the new, awesome-looking IOS that Apple will probably be showcasing via several commercials this fall.
I don't like it but I get it. Most of the general public won't though. With Android getting more and more popular I would think pissing people off in this way wouldn't be in Apple or AT&T's best interest.
So if it won't support IOS 5 at least let 3GS people who bought the phone in the first half of this year (or something like that) get a discount option for the new iPhone this fall if they so desire. If not they'll be locked into IOS until late 2012/early 2013.
EDIT: Ok I'm happy to say I was wrong. IOS 5 will support the 3GS. Way to go Apple!
-PN