Thanks to aStRaLgOd, who beat me to it with a post in BPN -- and also to jerichohol for providing me with a link to a fast mirror.
A patch for UT 2003 has been released. It makes (mostly minor) changes to almost all technical aspects of the game and is a required download for all UT 2003 players. Currently a Linux version is not available, but the Windows client and server are.