It's not like the United State's largest carrier isn't GSM.
Regarding using unbranded phones, after changing the SIM, voice and SMS will work, but configuring data and MMS on a non-carrier branded phone is a tedious process, and if you run into problems, other than giving you the proper settings, the carriers don't offer support.
i.e. If you correctly type in T-Mobile's MMS WAP IP gateway, APN name, and Messaging Server and your non-branded phone can't send MMS, they won't offer additional support.
Below are typical steps to get a data connection setup on a non-T-Mobile phone. I just can't imagine Apple selling unlocked, unbranded phones and expect the owners to have to type this in to get their new iPhone to connect to the Internet on T-Mobile, or type something similar to get it working on Cingular.