Well it seems that there's been quite a bit of confusion about DOOM3 and specifically the multiplayer. As John Carmack mentioned in his speech they were looking at a 4 player limited peer to peer system for DOOM3 currently. That is just what is currently working on DOOM3, however it has been clearly stated in the past, and again to me while at QuakeCon by a couple of employees that there are vague plans for a multiplayer follow-up patch or product for DOOM3 that have not been fully decided on. So dont panic! all this means for multiplayer is that the singleplayer version of DOOM3 may be including a limited multiplayer portion although probably not. Just keep your pants on and wait for the multiplayer patch/expansion. You can see the GameSpot folks mentioning this as well:
id does have real plans for more serious multiplayer options for Doom III, which Carmack said would come either in a separate product, perhaps as an expansion pack.
FileShack hosted MP3 of John Carmack's QuakeCon 2002 speech. Thanks monstaquake. This MP3 is missing the last part of the Q&A that covers that multiplayer quote sadly, if you have a full recording we can host please drop me an email.