I live in Northern VA (Reston area), and I made the switch to AT&T with the iPhone. To be completely honest, I am dissapointed with the AT&T service and coverage. I used to have strong coverage both indoors and outdoors with Verizon, and have noticed that AT&T's coverage is much more spotty. Additionally, I don't know if this is just a GSM thing, but the signal seems a lot more fickle, even in areas with strong signals.
Despite all this, I am very happy with the iPhone. The user interface and functionality make up for the lack of coverage (at least in my opinion).
It's a tough decision to make. If possible, find someone who has an iPhone or AT&T service and see if they can spend an hour with you examining the signal in your area. Hell, if you're close to the Reston area, let me know ... I'd be happy to meet with you and check the signal.