The free ‘Exterminatus’ DLC that adds limited co-op play to Warhammer 40,000: Space Marine has been released.
It has on Steam and through PlayStation Network, anyway. The Xbox Live version “will go live soon”.
Exterminatus adds a survival co-op mode which pits squads of four marines (Tactical Marine, Devastator or Assault class are available for each) against waves of incoming foes (Orks, most likely).
Any experience earned in this new mode will count towards the general multiplayer rank progression.
Have a look at Exterminatus in motion, below.