Akil "Lyndro" Hooper has posted a followup to his interview with Piestro on the official forums:
I know that much of this is information that you already knew, but we wanted to get everything in a centralized place, so people don't have to dig around to try and find the answers to these questions.
The AA requirement won't be 320 for sure (We don't want to require AoD), the bar for earning XP in 90 is going to be 280, because you have your heroic endlines at that point (and I hate large primes so 277 wasn't going to work). The requirement exists so we (the developers) and you (the players) have a little more information about what a level 91+ player is. Right now if you form a pick up group, you really don't have an idea whether your tank has 65 AAs or 317 AAs. The same goes for us, content for a level 90 with few AAs is pretty different than content for a level 90 with a whole lot of them.