1up put up an interview with Tim Willits and Todd H of Id software. The talk is mainly about Rage, but also touch on subjects such as why Doom is made an in house project. But this little snippet caught my eye in particular:
TW: We're not talking multiplayer yet. Our goal is to focus more on cooperative [gameplay] than competitive [gameplay]. We've got Doom and Quake, and we have all these other IPs that have competitive multiplayer. But for Rage, we want to focus more on cooperative.
That Rage was going to have a Cooperative game mode, was already known. But this suggest that it'll be more than just an afterthought.