The phone will work with any GSM carrier if unlocked but 3G will only work with Tmobile or other carriers that share the same 3G bands. If you're on AT&T you'll still get EDGE but no 3G. Rumor is that a phone with Moto Blur will be the debut android phone on AT&T but until that comes out, no 3G android for AT&T customers, unlocked or not.
You don't know this for sure. The rumor floating around from a reliable sourse says the unlocked version will be at&t 3g bands 850/1900. Would be kinda stupid to have an unlocked t-mobile band version AND a locked t-mobile version. Itw ould be the same phone... duhh