This happens to me if I do something stupid (like say to myself "Fink can't possibly ruin my X11 install again, right?")... I just run the X11 installer from the CD (Panther)... it says it doesn't use any space on the disk, so I assume it just moves/renames/activates things that were deactivated by Fink during my last binge X11 trip.
What have you done since the last time it worked properly? Fink? DarwinPorts? An .mpkg file that was supposed to install an x11 app and dependencies? something else? nothing?