Actually, T-Mobile has many large markets where they re-farmed 2G spectrum for 1900 Mhz use. What this means is, You can take an unlocked iPhone, put a T-Mobile micro or nano SIM in it, and get HSPA+ 4G speeds. T-Mobile has a map of this somewhere (Google it). T-Mobile will even sell you a nano SIM to put in your iPhone 5 if you open an account (provided you live in a 1900 Mhz area).
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Here is one such map: http://support.t-mobile.com/message/181306#181306