It's just AT&T that have this control freakery (well, and Apple.) T-Mobile doesn't care (and, is also GSM so you can just take your phone from AT&T straight over). Verizon doesn't care. Sprint doesn't care. Locally, I Wireless and US Cellular don't care either.
Unfortunately for the GSM users here in the states, AT&T and T-Mobile are both absolutely glacial in installing 3G, it's very common to have EDGE only from both unless you're in a big city. (We don't have T-Mobile here, but I Wireless JUST upgraded to EDGE last year!) In contrast, Verizon has a huge network that's nearly all EVDO, Sprint has almost all EVDO (maybe all EVDO by now), and US Cellular (the regional carrier) has blanket EVDO throughout the region. I'd love to get a GSM phone but unless you live in a big city, the CDMA/EVDO carriers are cleaning GSM's clock here in the states. Here's hoping LTE comes out fast enough that I can just get an LTE phone fairly soon (Verizon is planning a very agressive LTE rollout, AT&T is planning LTE, most carriers both GSM/UMTS and CDMA/EVDO are planning on LTE -- except Sprint who's got some Wimax rolled out.)