AT&T has EDGE everywhere they have 2G coverage - something that might be useful.
Also note that AT&T prepaid service does not permit roaming on other providers, so when you look at the AT&T coverage map, be sure to look at the prepaid map and not the postpaid one. There are huge areas of the US (particularly the central and western US) where there is little to no AT&T native service.
Then again, while T-Mobile prepaid permits roaming, it doesn't yet permit true data access, just limited WAP - and their 3G is at 1700 MHz anyway.
GSM: CA: SaskTel (postpaid voice/data,,postpaid data), Rogers (postpaid data); UK: Orange, 3; CH: Swisscom; AT: tele.ring; US: T-Mobile, AT&T, Red Pocket Mobile, Telestial Simple Calling; EE: AirBalticcard
CDMA: US: PagePlus
Hardware: iPhone 4, iPhone 3G, Nokia E63, Nokia Nuron, Novatel Ovation MC950D & MC998D data sticks, Huawei UG1691 data stick (3G AWS), Motorola RAZR V3, Sony-Ericsson K610i, Sony-Ericsson Z310i, Otech F1 (quad sim) (all unlocked)