iPhone 5, T-Mobile, GSM, and international travel
I switched from T-Mobile to Verizon in order to get the iPhone 4. I despise AT&T with a passion. If the acquisition is blocked and Apple decides to release two separate models of iPhone 5 -- a GSM model and a CDMA model -- then I will gladly switch back to T-Mobile. For people who travel internationally, an unlocked GSM iPhone is a huge asset, as it allows us to use local SIM cards in any country we travel to. We get a local phone number and don't have to pay any international long distance and international roaming fees.
The iPhone 5 will definitely support T-Mobile's 3G bands, whether Apple decides to make the iPhone 5 available to T-Mobile or not. We know that because the Verizon iPhone 4 contains Qualcomm's MDM6600 chip, which supports all CDMA and GSM 3G bands, including T-Mobile's 1700 MHz AWS band. (Of course, you cannot use the Verizon iPhone 4 with T-Mobile, since the antenna wasn't designed for it and also because there is no SIM card slot.)