Probably will be compatible. It's compatible with LTE from just about every other carrier.
According to the CNET livestream the answer is no. But they did mention that the LTE is GSM backwards compatible with HSPA+ which T-Mobile supports so who knows.
I believe they mentioned the 4 and 4S will work by the end of the year , I could be wrong
Apparently T-Mobile's LTE will be compatible with the iPhone 5.
http://www.tmonews.com/2012/09/t-mo...upport-hopes-to-have-nano-sim-by-mid-october/
So by mid 2013 T-Mobile users will have LTE and HSPA+ capability on their iPhone 5's. Just won't have official support from Apple or T-Mobile yet though.
AT&T iPhone 5 will work with T-Mobile's Future LTE network.
On evidence of that comment, I am assuming you're talking about the USA, apologies for wasting your times guys!