Trent Reznor conducted an interview recently with a magazine called "Rip it Up", and amongst other things, he talks a bit about his decision not to contribute to the Quake III Arena soundtrack. Here's an excerpt, spotted on SeemsLikeSalvation (by way of FGN):

On the Fragile I realised that when I work on tracks I automatically create a visual scene or image in my head and then just dress the set with music, I try really hard to work on the mood and flavour. What I would probably find the most artistically satisfying right now if David Cronenburg called me up and said "score a film, don't put pop music in it, actually score it." I would drop what I'm doing and see if I could do it. ID software, the guys I did Quake for asked me to work on Quake 3 but it was kinda like rock songs and it didn't interest me, I told them if you're ever going to do something that's interactive and dark and you need a real soundtrack then let me know. I would not do another lost highway, I'm tired of that. There's a whorish nature to putting pop music on a soundtrack, just to make a compilation record to sell. With Natural Born Killers, the use of music as a collage really intrigued me, it was a lot of work and I'm really proud of that one.