The 6630 will support WCDMA (=UMTS =3G) networks and GSM/GPRS networks at 900MHz, 1800MHz and 1900MHz.
It will not work in USA in regions with only 850MHz GSM/GPRS network coverage.
I have no idea if any US carriers are going to have this, but where ever you buy one (without a SIM-lock), the above statemens tell where you can use it.