In an interview over at Telefragged, Randy Pitchford from Gearbox gives us some hope that even if Halo for the PC doesn't have cooperative play at launch, it might gain it later on:

Randy Pitchford: Co-op for the Xbox wasnТt networked -- it was two view ports into one game. We can easily support cooperative play via split screen, but thatТs kind of a cop out. We want to do it right. ItТs important to us and weТre working on it, but we havenТt been able to promise that itТs going to be there at launch.

Thanks to Louis Wu at HBO for the heads-up.