GSM is the cellular network used by US vendors Cingular and T-Mobile? for voice and data communications. The data services are known as GPRS (General Packet Radio Service) (19.2kbit/s), EDGE (Enhanced Data rates for GSM Evolution) (200kbit/s) and UMTS (Universal Mobile Telecommunications System) (2mbit/s). The data services are an additional charge based on kilobytes sent or received or unlimited for a month depending on the plan chosen. Currently only Cingular offers UMTS in the US.

Windows Mobile Pocket PC and Smartphone are both available with GSM from Cingular and T-Mobile? as well as manufacturers such as HTC.

Links
Cellular Data Primer