Seriously research AT&T before locking into a two year contract. I've never heard good things about them and it's never a good idea to switch carriers just for a phone (even thought it is REALLY cool). I'm sure you have other reasons besides just the phone, though. Some of the other carriers are bringing in phones comparable to the iPhone and soon they will all have something like it - they have to keep up with the competition.
I've been happy with Alltel and will never go to Sprint or to AT&T. I hear good things about US Cellular, unless you travel quite a bit. Consumer Reports and similar magazines do comparisons of all providers, you might want to look into that.