I had a similar problem a few weeks ago. It seemed to happen after I installed a beta version of some s/w - don't even remember what it was now, just that is was the only version advertised to work in 10.6. Ha.
Anyway, first I tried a regular reinstall of 10.6. No dice so I went nuclear & wiped my drive and did a clean install then reinstalled data, apps from my backup drive via Carbon Copy Cloner which did the trick. However DO NOT reinstall any user or system preferences or you will end up back at Point A. I'm sure there is an easier way, but I'm only a Level 2 Geek, so this was the path of least resistance for me.