A community manager on the game's official forums posted over the weekend in what appeared to be an attempt to clarify things that the co-op mode had indeed been delayed by 30 days post-launch, but that it would be free to those who purchased the game "brand new." That suggests an online pass-style setup, not a new trick for THQ. But that forum post was itself deleted (cache here), suggesting that it too wasn't the real story.
So we went to ask THQ ourselves, and the official word from the company to Joystiq is this: "Warhammer 40,000: Space Marine will have co-op. We are looking forward to announcing more details later this month!" There you go. Co-op confirmed. As to how it will be implemented, we'll all have to wait and find out.