Nope. Apple updated the iPhone to add the AWS frequencies that Tmo uses alongside the same frequencies it's always been using for AT&T. Nothing has changed network-wise (not counting new plans and such)
So the 920? Same situation it has been as of late: you have to wait for them to "refarm" the frequencies in your area so that they use the 1900 band that the 920 and all AT&T phones use. If you hear of someone getting 3G/4G on their current unlocked AT&T iPhone on T-Mobile in your area then your 920 will work. It seems to be a very slow process though.
While not official, and likely not completely exhaustive since everybody would have to know about this site,
www.airportal.de has a map where users report areas they've found refarmed bands in.