Mind also that some phones support some of the frequencies but not all. For example, I have a Nokia Nuron that supports UMTS 1700 (AWS) which T-Mobile USA uses, and UMTS 850 which AT&T, Rogers, Bell, Telus, SaskTel and MTS use. However, in some areas there is 1900 MHz UMTS coverage and not 850, and in some areas you'll get better service having both 850 and 1900 even though an 850-only device will work.
GSM: SaskTel Canada (postpaid voice + data), Rogers Canada (postpaid data), T-Mobile US, AT&T US, Red Pocket Mobile US, Orange UK, 3 UK, Swisscom Mobile Switzerland, tele.ring Austria, airBalticcard (Estonia)
CDMA: PagePlus US
GSM phones: iPhone 4, iPhone 3G, Nokia Nuron (AWS 3G), Otech F1 (quad-SIM 2G GSM), Nokia E63, Sony-Ericsson K610i, Sony-Ericsson Z310i. All unlocked.