As promised, four-player online co-op is here for the PlayStation 3 and PC versions of Warhammer: 40.000: Space Marine, with the Xbox 360 version following soon after. And so the enemy of my enemy becomes my friend.
Relic calls it Exterminatus mode, and it adds two new maps and a standalone game mode to Space Marine's menu. Four players team up online to face off against the Greenskin hordes, killing baddies, completing challenges, and racking up the points so someone still comes out on top.
I'm re-downloading my Steam copy of Warhammer 40,000: Space Marine as we speak. Many Orcs will die this day. Just not on the Xbox 360 (it's coming soon!).