I think you'll be able to put in a T-Mobile USA or AT&T USA SIM card and register on those carriers' networks without any issue, but I'm pretty sure you'll be limited to EDGE data speeds.
T-Mobile USA I think requires either 1900 UMTS or both 1700/2100 UMTS bands. From: T-Mobile USA - Wikipedia, the free encyclopedia
AT&T USA uses either 850 or 1900 UMTS bands. From: AT&T Mobility - Wikipedia, the free encyclopedia