Hi! Welcome to the forums.
As far as I'm aware nothing has been mentioned of playing as the Aliens. If this changes I'm sure we'll hear about it, as it's something a lot of people want to see. However, I have a feeling Gearbox are going to make a pure co-op game.
The idea of playing as the Aliens and stalking a marine team ala Left 4 Dead would be a fnatastic add. If it's not in the main game, we can expect a mod, no doubt.