The cooperative mode for Warhammer 40k Space Marine has been released for PS3 and PC today, Xbox 360 update to come later. The mode is called Exterminatus, an arena mode that puts you and three friends against unending waves of enemies. Your characters are persistent and everything from multiplayer carries over.  You may also complete unique challenges to get the team some extra points. All experience earned will carry count towards your multiplayer characters progression. There isn't a lot to this mode, but it is free and it is cooperative, but it's not cooperative campaign or missions like many were hoping for. Check out the release video below and for more information on it check out the main website.
More Exterminatus Information