There will be multiplayer on Warhammer 40,000: Space Marine's disc when the game comes out next week, but there won't be co-op.
Instead, that's coming to the game as free downloadable content sometime in October, the developer confirmed today.
This isn't a separate co-op campaign or anything, but a Horde-like, score-based four-player mode called Exterminatus. Players choose between three classes--Tactical Marine, Devastator, Assault Marinefight-- and fight through two different scenarios--Assault on Hab Center Andreas, Escape From Kalkys Facility--that both feature multiple maps to slog through.
It sounds pretty involved, actually.
You'll earn experience within Exterminatus that can be applied to the other multiplayer modes, too.