I'd say it would be a bad decision to switch to ATT just for an iPhone.
First, you need to spend some time at
#attfail. These are real-time Twitter complaints about ATT. It is among the worst carriers, period.
Second, I know a fair number of folks with iPhones, a few with iPhone 4. None are blown away with it. Reception has always been spotty with iPhones, it's worse with the iPhone 4, and call/sound quality is mediocre at best.
Yes, this is anecdotal. But do some searches yourself, you will see a
lot of people saying the same thing.
You're on Verizon now? I guarantee you -- guarantee -- the Verizon network is
vastly superior to ATT, in terms of coverage, reception, and reliability.
So if you want to pay more money for an inferior network so you can tool around with an iPhone 4, make the switch.