User blog comment:Commdor/Rumor: ME3 to Feature 4-Player Co-op Missions/@comment-3578615-20110618151847/@comment-1655340-20110618213351

It says that a it was untrue that a multiplayer spin-off was going to be announced instead of ME3 (which is really just a paraphrase of what Commdor said, but whatever).

However, like Commdor said, they didn't rule out the possibility. It won't happen in ME3, but that doesn't mean it won't happen.