Originally Posted by Adar
You won't be able to get the update then. T-Mobile hasn't released it yet. Even though the phone was originally AT&T, since it is now T-Mobile, you won't be able to update it. Well, not without rooting again and using some other ROM. You'll have to wait for T-Mobile to release it.
I don't think this is correct... historically, you'd get the update when your original carrier released the update. Even though your radio is unlocked, your phone is still running your carrier branded version of android, so you should get the update when your original carrier (AT&T) releases the update.