3G is a data connection, not voice. Voice uses plain old GSM.
3G may work in the USA, using the usual roaming system, but GPRS certainly works over there and is 2x faster than here. Any 3G data module should fallback to GPRS.
On PAYG, say UK Vodafone, the rate is stiff, around £10/MB, but it does work. I found Vodafone and Virgin (T Mobile) both worked fine on GPRS.
GSM of course works fine anyway (voice calls).