Gamers.com have posted a interview they had with John Carmack about Doom III - The Legacy. Here's a byte from it:
SD: The Doom series also has a substantial multiplayer legacy behind it. What can we expect Doom III to add to that legacy?
JC: This has been one of the areas where I know we are going to get some negative feedback. Doom III will only have minimal multiplayer facilities when released, because we are concentrating all of our efforts on making it an outstanding single-player experience. We have our hands full just breaking in the new technology and creating our world, so there just aren't enough resources to go around for doing spectacular multiplayer. You will still be able to deathmatch, but don't expect anything revolutionary. We will certainly be doing follow-up titles that refocus on multiplayer. I strongly believe that it is better to do more games with a tighter focus, rather than trying to be all things to everyone in a single title. DOOM III John Carmack Interview @ Gamers.com
SD: The Doom series also has a substantial multiplayer legacy behind it. What can we expect Doom III to add to that legacy?
JC: This has been one of the areas where I know we are going to get some negative feedback. Doom III will only have minimal multiplayer facilities when released, because we are concentrating all of our efforts on making it an outstanding single-player experience. We have our hands full just breaking in the new technology and creating our world, so there just aren't enough resources to go around for doing spectacular multiplayer. You will still be able to deathmatch, but don't expect anything revolutionary. We will certainly be doing follow-up titles that refocus on multiplayer. I strongly believe that it is better to do more games with a tighter focus, rather than trying to be all things to everyone in a single title. DOOM III John Carmack Interview @ Gamers.com