I have an unlocked Vibrant, and it works well on AT&T's 3G network. Generally it's AT&T phones that don't work well with others, since they actually disable non-AT&T frequency bands.
Interesting, all the specs I've seen say it only supports UMTS Band I (2100, Europe) and IV (1700 "AWS", US T-mobile). You'd think they would have mentioned that it also supports band II (1900, AT&T). I just ran a fresh search and came up with this page which supports your claim, but it's absurd not to list it anywhere else: http://ars.samsung.com/customer/usa/jsp/faqs/faqs_view_us.js...