The Bold is the premier BlackBerry offered by AT&T, and the pinnacle of achievement for RIM, IMHO. Your unlocked 8900 will do fine on the AT&T network, but you won't get 3G as the 8900 isn't built for it. And no UMA as Midnightdraven said.
Sorry guys, blackberry i'm talking about is 8100 (pearl)
Sorry for confusion, i'm asking this for my co-worker but since i got pearl as well i joined the forum for "future"