Icculus posted an Unreal Tournament 2003 patch announcement. This patch brings UT2003 up to version 2136. The easiest way to update this is with loki_update, which is included in the updater/ directory of your UT2003 install, but you can also download the patch directly.
The full changelog and mirror list is available here.
Update: For those of you having problems with glibc, mostly Debian Unstable users, read the posts below this bug report.