Twice recently (just yesterday and today) I experienced a problem where keyboard shortcuts - like Dashboard or Spotlight - stop working, and when I try to logout or restart/shutdown, OS X won't do it. Apps quit but it just sits there and I have to cut the power. The last thing I installed was the Safari 4 beta, but that worked fine all week before this started. I can't see any processes that are hanging, I've added no new peripherals, etc. (bottom line, I've not changed anything, so I don't know what's causing it).