DICE has kept quiet on Battlefield 3's co-op mode until now; a new interview with executive producer Patrick Bach reveals it's two-player and will feature its own missions and objectives separate from the single player, as built by a separate team.
Missions "string together in an interesting way" with single player missions, but how is still yet to be revealed. As for maps, these will take in part from single and multiplayer for the sake of integration (not re-use), but for the most part will be unique. Finally, both infantry and vehicle play are confirmed.
The two-player count is a bit of a letdown, as it's easy to imagine plowing through with a proper four-man squad similar to what we've seen in the single player footage, but everything else is sounding just darling.