My iPhone 4 is on 1.59.00 and unlocked and my Tmobile regular plan works i just had to cut the sim and it worked. However I had an iPhone 3G a while back and had a prepaid Tmobile sim card to see if my 3G was really unlocked and incoming and outgoing calls and text messages worked fine.
ATT uses go phone as sometimes the sim only works with the go phone. However with Tmobile you should be good to go.