IF THE ENGLISH MAC UT2003 INSTALLER CRASHES AFTER YOU ENTER YOUR CD KEY: Then please download this.
You should end up with a disk image with an installer icon. Run it, and install as usual. When the installation is done, throw away the disk image and pretend nothing ever went wrong. This is an installer bug; the game itself runs as usual.
The retail disc installer apparently calls CFRelease() on an invalid pointer (whoops), which has caused a most unsatisfying crash for a small percentage of people. We never saw this one, but those that trigger it seem to do so reliably. The French CDs have this fix already, so this is just for the English retail version that shipped on June 11th if you crash for no apparent reason after entering your CD key.
(This is not really tested, since we couldn't reproduce the crash, though. reports of success are welcome.)
This is only needed if your UT2003 installer unexpectedly quits after you enter the CD key. If you don't experience such a crash, then you don't need this fix.
Make sure you have your UT2003 CD in the drive before you try to use this installer fix. Then run the installer on the ut2003-mac-setup-fixed.cdr.dmg disk image (not the one from the UT2003 CD). The installation should then proceed as normal. After the game installs, you can trash the disk image.