Yep: it's online. BioWare's detailed how Mass Effect 3's four-way co-op will impact the main campaign (optionally), how development affected single-player (it didn't), and how many Shepards you can have running around (none).
In a post on the BioWare forums, Chris Priestly revealed that the co-op missions, while not not part of the solo experience, "will have a direct impact on the outcome of the single player campaign, giving players an alternative method of achieving ultimate victory" against the Reapers.
This ability to influence the outcome of the story is called the Galaxy at War system, and will impact your solo campaign's "Galactic Readiness" level.
The co-op missions apparently form just one part of this system, with "other platforms and interfaces" to be announced soon.
"It is important to note that the system is entirely optional and just another way players can have control over your game experience – it is still possible to achieve the optimal, complete ending of the game in Mass Effect 3 through single-player alone," Priestly assured readers.
In an extensive FAQ, BioWare addressed common criticisms of the the undetailed multiplayer, commenting that it had gone for co-op over versus modes so that multiplayer "made sense and did not compromise the power of the single player campaign".
"Our priority and focus with Mass Effect 3 has and always will be to deliver a complete and satisfying single player experience," the post reads, going on to comment that no development resources were reassigned as a result.
"A dedicated team from our recently formed BioWare Montreal studio has been focused on creating the multiplayer game features while the main game continued to be developed by the team in BioWare Edmonton. Both teams are integrated under the same leadership group that produced Mass Effect 1 and 2, led by Casey Hudson."
Speaking of leadership, the beloved Commander Shepard does not make an appearance in the multiplayer missions.
"Commander Shepard’s part in the war will take place in the single-player campaign, as will that of other beloved characters in the franchise such as Garrus, Ashley, and Liara…these characters do not appear in the multiplayer missions.
"In multiplayer, players will create custom characters to fight on different and unique fronts in the war. This will include the ability to play as favorites like Turians, Krogans, Asari and more… each with their own unique set of abilities."
Mass Effect 3 is due in early 2012 on PC, PlayStation 3 and Xbox 360.