"Leo M." reports:
"Like some of the readers out there, I updated to 10.5.7, and had mdworker go crazy, spiking to 130%, messing up Time Machine backups, etc.As always, when removing items like codecs or .plist files, drag them to the desktop first to test. If the solution works, you can then delete them. Of course, always have a recent backup ready to go incase you lose information.
I tried restoring from backup, re-installing the Combo, all to no effect.
The 'culprit', believe it or not was older  Xvid Quicktime plugins. Even though I had 'Movie / Media file' searching unchecked, mdworker was still trying to access these files. And hanging. Found this out via Activity Monitor > mdworker > open files and ports.
So if your users are having issues, look to your /Library/Quicktime folder if all else fails. Removing these older plugins helped in my case."