I ended up repairing permissions using Disk Utility and it turned out that the files in the iTunes package needed a bunch of fixing. Since then it's doing okay.

I've also found that repairing permissions after OS updates helps a lot as well.

Apple still hasn't quite gotten this "UNIX with a friendly face" thing down yet, but as long as there are tools like [link|http://www.versiontracker.com/dyn/moreinfo/macosx/18282|Cocktail] I'm willing to live with a little periodic maintenance.