Looks like you'll only be able to get your iPhone from AT&T Cingular AT&T for the next five years. Of course, you could always buy it from AT&T, then pay to have it unlocked, then go to the carrier of your choice...

"Being an AT&T-exclusive means that the iPhone will remain a GSM phone, leaving it clearly out of reach for CDMA wireless users. Verizon Wireless and Sprint are both CDMA shops, and customers of those carriers who wish to get the iPhone will have no other choice but to make the move to AT&T."