id Software's Timothee Besset has announced the release of the Linux client/server binaries for Doom 3. A Linux Doom 3 page has been setup with installation instructions and a FAQ. One item of note:

Currently, the game will not run correctly on ATI cards using the fglrx driver. However, the ATI developers are working on new driver releases, and eventually the game will be supported.

A demo has also been released if you haven't had a chance to purchase the retail edition yet.

Official:
http://zerowing.idsoftware.com/linux/doom/
Download:
http://3ddownloads.com/Action/DOOM 3/Patches/doom3-linux-1.1.1282.x86.run

Fun stuff.