http://www.thesixthaxis.com/2011/10/11/mass-effect-3-gets-4-player-co-op-missions-galaxy-at-war-system/
“Success in multi-player will have a direct impact on the outcome of the single player campaign, giving players an alternative method of achieving ultimate victory against the greatest threat mankind – and the entire galaxy – has ever faced.”
There better not be story related content which is only viewable through doing this multi-player crap. Maybe it's like say you needed Wrex on your side, instead of doing his mission in single player you do it in multi-player? I could be clutching at straws....thanks Bioware.
EDIT: Maybe it's not as bad as I thought. http://www.oxm.co.uk/34661/mass-effect-3-multiplayer-update-no-shepard-no-versus-and-not-compulsory/?cid=OTC-RSS&attr=OXM-General-RSS