I am using an unlocked, unbranded 9900 white on AT&T (bands 1,2,5,6) works great. Haven't tried VVM yet but I heard it's possible. The other interesting thing is you will be able to use OS 7.1.0.523 (at least I can).
I can use 7.1.0.523 also, and I have a locked AT&T branded phone on their network. No battery life problems whatsoever, and no mic issues or anything. It's being a dream phone.