In reply to steve taylor:
GSM is split into two bands in europe, GSM900 and GSM1800 - these are split between the various network operators in each country. In America there are two mobile comms bands, the 850 and 1900. Unlike europe these are not universally GSM so you wont necessarilly get GSM coverage everywhere in the states but where GSM is covered you will need either a tri-band (which covers GSM900,1800,1900) or a quad band that covers all four. The advantage of a quad band phone is that you may get better reception from an 850 operator than 1900 so coverage is better, but this is not essential. For full coverage in the states you will need a phone that can cover CDMAOne aswell (but you will find it hard to get hold of one of these in the UK).