According to the Apple rep I bought my TMO iPhone from the answer is yes. Everything I've read says yes. I've never read anything contrary. But, insert the ATT sim and find out!
I found a rose gold T-Mobile iPhone at an Apple store in Maryland today... reserved it and picked it up at 5pm. Yes it's unlocked... she put my sim in right there at the store and I didn't have to do a thing. But if I didn't have my sim she said once I came home and restored from backup I would have gotten the "your iPhone is unlocked" message.