At least not at launch, will likely come in an update
If you are a Warhammer 40,000 fan and have been waiting for the release by THQ of Warhammer 40,000: Space Marine you are likely not going to be happy with some of the talk that happening on the official forums. Apparently, while the posting was removed, the word is that multiplayer Co-Op for the game is off the table at release.
The rumors seem to indicate that while the developers are committed to the multiplayer Co-OP and they still plan to add it, they need more time to do it right; and with the crunch to finish the game right now the effort needs to go into finishing the main game.
The word is that the multiplayer Co-Op will follow the release of the game in about 30 days after the game ships in an update of the game. THQ has not confirmed that this is officially the way it is going down, but it seems (despite the removal of the post) those in the forums believe that this will be the case.