The guys at GameSpot had the opportunity to chat with id's John Carmack a bit and of course they asked about DOOM 3. The topics covered in the article are character animation, the save system and graphics scalability. Here's a tidbit

Carmack said that scaling Doom 3's resolution will be the preferred way to increase performance without sacrificing graphical effects. It's undecided whether he'll allow players to turn shadowing on or off, since the shadows are essential in many of the game's dramatic scenes. Since the shadowing accounts for about half of the game's render time, though, he said he'll probably include the option