You know, I think I've actually had this problem some years ago. I.e. it was not possible to shut down the computer from the Shut down dialog, you would just be returned to LS once you selected something, and you had to kill LS to be able to shut down. Unfortunately I can't seem to remember what was causing it -- all I know is that the problem went away. Could have been a tray app like Boreas says. I'll let you know if I remember what it was.