http://social.bioware.com/forum/1/topic ... /8481789/1
Some key points: the single player campaign is separate and if you want you can opt out of multiplayer co-op entirely and you won't be adversely affected (they say that you can reach the optimal ending for ME3 just on the single player campaign).
Co-op mode is 4-players, you can play as custom characters including the aliens (especially including krogan), and what you do in the co-op missions can affect the single player campaign. I don't think there's any word yet whether you need 4 players or if you can play with 2 or 3 (I'm guessing you can but who knows).
My reaction: I'm fucking loving this news. I want to play ME3 RIGHT NOW.