It also helps that in the UK there is only one radio standard for cell phone communications....GSM.....in the US we have GSM, CDMA, and PCM......iPhone is GSM, so you can only use AT&T or T-Mobile.
This was true in Canada, up until last year. Bell and Telus migrated from CDMA to 3G GSM in November, 2009 (their CDMA networks remain running but will be shut down once customers are migrated in a few years). SaskTel in Saskatchewan migrated similarly in August, 2010. MTS in Manitoba is migrating later this year. TBayTel has already migrated. There may be one or two very tiny regional carriers still remaining.
One tiny new carrier with spectrum in the 1700 MHz band has elected to use CDMA (for really peculiar reasons; why would you want to embrace a dead-end technology on a new network when you could use 3G GSM and have roaming capability worldwide?), but the other two that have launched on that band are using 3G GSM.
Fido was the first GSM network in Canada (1900 MHz); Rogers converted to 850/1900 MHz GSM in about 2002 and bought Fido a few years later. Some coverage is still 2G, but much is 3G.
So, in a few months, all Canadian networks except for one unimportant one will be 2G or 3G GSM No wonder Fido sells unlocked iPhones here! If a future iPhone supports 3G 1700 MHz, all carriers but one will be able to use iPhones here.
The US, for whatever reason, is different. I understand that it would be a lot more work to convert a huge network like Verizon's to 3G GSM, but if Verizon did, it would have a lot more choices for phones and Apple would surely end the exclusive arrangement with AT&T. If T-Mobile deployed 3G service at 1900 MHz (it's currently 2G-only on that band), the same might happen.
Nowhere else on Earth is this is an issue.
