Last time I was there it roamed onto at&t but obviously as I'm not bill gates couldn't afford to use the data service there.
Depends on handset, but most likely for you to get 3G service in the USA is AT&T who are huge and national. Unless you have a really old phone.
T-Mobile is the only other network you will be able to connect to, but in many areas its 3G frequencies are not supported by european phones. (they use the AWS band which is 1700 for 3G - except in some areas where they have 1900 3G as well).
AT&T use 1900 for 3G.
They both have higher speed 3G as we do here (H symbol on Android) but both foolishly call them 4G, not to be confused with the real 4G services called "LTE" in the US.
James BT Infinity 2
19/09/2012 - Sold 42/6 - Getting 49/8.5 - Sync 53 / 9.5 Mbps @ 470m approx
14 years of broadband (ntl: cable to BT FTTC) - Router: Asus RT-N66U - Modem: Huawei HG612 speedtest