Just wanted to confirm that the iPhone 5 is working on T-Mobile. I purchased a full price version (AT&T 16G) from an Apple store on launch day. I specifically said I needed it not to be under an AT&T contract. When purchasing, they processed it as "Device Only" and all was fine. On the receipt it says - "No Wireless Service Activation during iPhone Sale". I even have registered AppleCare.
I did a hack job on a spare T-Mo SIM (regular size) I had sitting around that was never activated (you can order them online). I used the nano SIM that came in the phone (AT&T as a guide). First I cut with scissors and then fine tuned using sandpaper. I just kept at it until it fit in the SIM tray for the 5. I called T-Mo and had them transfer service from my existing Micro SIM in my iPhone 4 to the modified SIM in my 5. I was too afraid to try to use my existing SIM for this so was glad I had a spare. I did *not* have to sand SIM for thickness at all. I have restored from back up via iTunes and have called out, sent SMS, MMS, etc. all functions seem normal. So some of the T-Mobile specific info for set up may have come from the restore info on my iPhone 4. Most of the proper set up info can be found on the T-Mobile set up docs for iPhone.
Now that I am using it, so far so good. I have seen Edge data work fine. This week I am travelling in a city that has had reports of refarmed 1900 MHz T-Mobile towers. I just got here but plan to travel to those areas and see if it works for me or not. Info can be found here on those reports:
I will try to let people know how it all goes.
EDIT: I should also add that I had been using T-Mobile's Bobsled app for calling over wi-fi. However under iOS 6 that seems to be broken. It makes calls but the other end is unable to hear me. I have tried numerous times and changed what I could, but still no go. I hope they address this soon.