Bioware has finally confirmed the game's co-op mode in a Q&A on its forums.
The new (optional) four-player missions take place outside of the main campaign, and aren't affected by your previous Mass Effect progress. "Players can choose from a variety of classes and races, form an elite Special Forces squad, and combine their weapons, powers and abilities to devastating effect as they fight together to liberate key territories from enemy control," the official blurb reads. Apparently the multiplayer will tie into your single-player progress, offering an "alternative method" of achieving success in the main storyline. Curious.
But there's more. The multiplayer ties into a feature called "Galaxy At War", an additional, optional system that uses multiple areas of the game to depict Shepard's "Galactic Readiness" level. Hmm. "Other platforms and interfaces will be announced in the coming months," Bioware said.
The mystery persists.