Don't Try This on the 3G iPhone
T-Mobile US Inc. is the only other operator running a 3G GSM-based network in the U.S., and they only cover New York on a 1700 MHz frequency that no other service provider yet supports.
So check the box and see what bands the 3G iPhone can run over before expecting to unlock the phone. If 1700 MHz isn't there, then the phone won't work on any other net apart from AT&T's in the U.S.
This is not true of the rest of the world, however. In fact, I wonder how many people are planning shopping trips to pick up a cheap 3G iPhone when they become available in the U.S.
— Dan Jones, Site Editor, Unstrung