I picked up a iPhone 4 32gb 4.3.1 off Craigslist and purchased a Gevey Sim Pro from eBay. Well, the Gevey Sim Pro works, used a microsim cutter then cut a little more to fit the sim card into the Gevey Sim Pro tray and after following the directions, within 2 minutes, I was using the iPhone 4 on T-mobile. Of course the only drawback is that its on EDGE but thats the price to pay to use any iPhone on Tmo.
Got MMS to work using the T-mobile MMS fix from Cydia.(must restart the phone to work, and that means going through the unlocking steps for the Gevey Sim Pro)
Facetime is not working. Tried several tweaks/suggestions/apps from Cydia but couldn't get Facetime to activate. For example, on another forum, someone claimed that Facetime was activated by editing your Phone number by adding a "+" or '1", then sending yourself a text while facetime is switched on and doing this over Wifi. You have to wait several hours till it will activate. Tried this four times to no success. So I've been using Tango for video calls.
I've been using the iPhone 4 for a month now and its a very good phone. My HD2 running Android is by backup phone. Will see this fall if I'll change to a new phone, the HTC Hercules and new WP7 mango phones are coming.