In the past, T-Mobile's 3G network would not function on the iPhone because AT&T used, and still uses, 1900MHz frequency band whereas T-Mobile uses 1700/2100MHz. The new iPhone 5 now supports HSPA+ 2100MHz. Therefore the new iPhone 5 will be able to use T-Mobile's 4G network as long as the phone is unlocked and the T-Mobile sim is cut down to the new nano size, correct? I'm pretty sure that this is true, but maybe there are some other obstacles to come over (besides unlocking it). If this is true, I will wait until the iPhone 5 is unlockable, then buy it for use on my T-Mobile network (right now I'm using the Samsung Galaxy SII).