User blog comment:Commdor/Rumor: ME3 to Feature 4-Player Co-op Missions/@comment-737801-20110616182455/@comment-1221773-20110616211727

It sounds to me like the co-op missions would be an extra mode, separate from the main single-player game. It brings to mind Red Dead Redemption's Outlaws to the End DLC, which added some co-op missions to multiplayer and had no effect on the single-player.