This is an update to the issues that have plagued the latest Mac OS X update, 10.5.7. Basically:
“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.”
Read the whole article, sounds like a good fix.