Today Expose stopped working. I also noticed that the Dock magnification didn't work (I normally hide the Dock) and the submenus under the Apple in the top-left corner wouldn't open.
First I restarted the Dock. No joy, so I restarted Finder. No joy, so I escalated to logging out. Still no joy, so I rebooted the computer. The problem is still there. My exocortex (Google) doesn't seem to know anything.
Then doing something else, I unplugged the USB hub and viola the problem is solved. A little investigation shows that it's my EyeTV Hybrid that's causing the grief. When I unplug it, Expose works fine. When it's plugged in, Expose will work once, then quit. I reinstalled the EyeTV software and the problem seems to have gone away.
What a weird interaction.