Also, nearly every carrier outside of the United States of America that offers the iPhone as a device under contract is either owned by AT&T (like Bell or Bell South) or has a contract with Apple like AT&T has which can exclude many carriers using GSM/GPRS/UMTS technologies. However, an unlocked iPhone is compatible with only GSM/GPRS/UMTS technology so goto:
http://en.wikipedia.org/wiki/Mobile_Network_Code to find your carrier and make sure that the network is compatible.