Bioware's main men Dr. Greg Zeschuk and Dr. Ray Muzyka talk about Mass Effect 3, Star Wars: The Old Republic, future unannounced games, and much more in an exclusive interview with Game Informer.
Judging by that it's obvious that ME3 is not the end of Mass Effect, so I'd have to agree that an MMO is a strong possibility at this point. I think they'll probably wait and see if TOR is a big success or not first and then make a decision.