Yep, the beauty of GSM. You can use any carrier locked AT&T phone or an unlocked GSM phone (in most cases the latter works, but if you happen to get an unlocked T-Mobile 3G phone of some sort, it wouldn't work on AT&T's 3G).
Small tangent. GSM phones are so cheap, I went with T-Mobile's Even More Plus plan, which gives you a no contract plan with a cheaper monthly rate in lieu of phone subsidization. You could get a fairly sophisticated Nokia e63 for $130 unlocked almost every month of the past year. Not in the same league as an iPhone, top tier Android, or high end BlackBerry, but better than the average feature phone. A nice low end messaging, email, Skype, chat capable smartphone.
Nathan