Hmm, this is a bit of an odd one, via ShackNews. This Space Marine forum post, which has subsequently been deleted, saw a THQ community manager claiming that the co-op aspect of Space Marine would turn up 30 days or so after release, but as then clearly been redacted. Since then THQ have released a statement saying: "Warhammer 40,000: Space Marine will have co-op and we look forward to announcing more details later this month."
I guess someone is in trouble for blurting out meticulous PR plans, eh? Tsk.