There've been plenty of details on how you buy an iPhone and sign up for AT&T service if you're not a current AT&T customer, but until now current users have been jabbing in the dark about what they have to do. Well, AT&T's finally put up a FAQ, and what a great FAQ it is. The most important detail is the fact that any AT&T customer is eligible, even if you've just upgraded your phone recently. And there are no penalties either.
For people already on board the death sphere, you'll have to go to an AT&T store and the retail employee will help you upgrade your phone. You'll be able to go upgrade through myWireless online later. You don't have to switch to a different plan—you just have to add the iPhone Data plan starting at $20.
But if you decide to return your iPhone within 14 days, you'll have to pay a 10% open box restocking fee.
iPhone FAQ [AT&T]