Gamespy has an interview with Robert Duffy who is the developement tool coder for Doom III, where he explains a few interesting things for modellers and mappers looking forward to producing things for this engine..
"In Quake 3 a model may have two or three thousand polygons, whereas the player models in DOOM III, the original art has probably close to a million polygons. That's been reduced down to four or five thousand for in-game use, but you have to have that million polygon model - you have to have a lot of polygons to generate the bumpmap data that makes it look like a high-poly model in game.
..you can set the two next to each other in-game and you can barely tell the difference, because of the bumpmapping and specular effects, etc."
"A large percentage of the textures are done exactly the same way. They'll model a wall at a very high polycount, and then renderbump it and turn it into a bumpmap set, into a set of textures that all combine together to give it the 3D look."