This is an old thread, but I think it's important enough to get an update.

The Linux game market is still an underdog but it's there.
UT2004 plays like a dream out of the box

I had to wait a few weeks after purchasing DoomIII for a Linux binary but I now have it and it's fantastic.

Neverwinter Nights also has a Linux binary but I've not bothered to get it running even though I purchased the game with the intent of running it under Linux.. beat it with Windows and didn't bother to install it.

A few problems still seem to plague linux games.
1, the installer. Developers often think that a Linux user is going to be a guru and will know exactly how to get a game running. Too many games rely on installing to a windows partition and copying the datafiles across and replacing the binary with a Linux version. I'd like to see this improve.

2, The trust. There seems to be the attitude that all linux users are freeloading scum.. I beg to differ. Of the windows users I know, there are many who think nothing of playing a pirated game they downloaded from the net. Maybe it's because there's not as much commercial software for Linux but I don't know many people who use ripped off software on their linux box and I know plenty of Linux users.

3, The market share. Software companies only develop if there's a market for their work. A platform has to be popular before a company will support it. But PC platforms become popular because it's generally what people use at work and they mostly use windows. However, this is changing. More companies are moving to Linux. My company has a significant Linux investment and the UK Government have recently completed a favourable study into the use of open source. So as Linux becomes more popular in the work place, it will become more popular at home. It is becoming easier to use too. As it improves, hardcore gamers will shift to take advantage of the benefits.

4, Drivers. nVidia have been supporting Linux for some time now but ATI seem to be lagging. John Carmack of iD software has had no end of trouble getting DoomIII running on ATI cards under Linux. For new games to be popular on Linux there has to be the hardware support. Games have to be able to run at least as quickly as their windows counterparts. This is slowly happening. But DirectX is very good even though it pains me to say it. it will take some beating.

Well, that post was longer than I thought it would be... but if it gets a nice balanced debate going, cool