From GameSpy: (http://xbox.gamespy.com/xbox/doom-3/512537p2.html)
"On the multiplayer side, the Xbox version of DOOM 3 will include standard deathmatch, 4-player team DM (2v2), and last-man-standing modes, played out over 5 multiplayer maps. However, maybe the most exciting component of DOOM 3's multiplayer is support for 2-player co-op via Xbox Live and System Link (no split-screen, sorry) -- which will not be in the PC version of the game. There aren't any plans for downloadable content or compatibility between the PC and Xbox versions, but the idea of playing DOOM 3 cooperatively with a buddy over the Internet could be a huge selling point for the game on its own. "
Well there ya go! 2 on 2 Garbage! YAY!
But I do think the Single Player is where it's at.
I'm a Goldmember!