I think the title says it all! I guess all the guys & girls at Linux.com are playing Quake now.
Enjoy the how-to here:
As soon as I read the news on Slashdot that id Software's just released Quake 4 was already available as both a Linux client and game server, I ran out and dropped $50 for the game. The box contained 4 CDs, a game guide, and a license key. But that's not enough to let you play Quake 4 on Linux.