My HD died a while back, and I thought I was all set with the replacement from Apple, but turns out it didn't have my X11 stuphs.
Kept getting errors like:
Jun 30 18:02:02 den org.x.startx: X11.app = /Applications/Utilities/X11.app
Jun 30 18:02:02 den org.x.startx: Version = 2.1.5 (2158000), Expected Version > 2.3.0 or 2.1.6
I don't know where the hell my install disks are, so I thought I was hosed till I found 'em.
Turns out you can re-install, and even upgrade, the Apple X stuff using the creators site: