There are only two GSM carriers in the US (AT&T and T-Mobile) and they each use different 3G frequencies. If you want an Android phone with T-Mobile 3G you might be able to import one from Canada's Wind network or Mobilcity, though I don't think you'll be very impressed with the phones they offer. Right now T-Mobile is the only carrier using the AWS 1700/2100mhz frequencies that has high end Android phones. If you want a phone that'll work on both AT&T and T-Mobile, as well as in Europe and Asia, you can buy the Nokia N8 but you'll have to settle on using the Symbian^3 OS.