I'm running a late 2008 Macbook (aluminum) and I have never had a problem like this in the past until after I applied the 10.6.4 update. Whenever I call up expose (through the 4-finger swipe down, F3 button, or the application itself) I only get a dim screen. None of the open windows are "grabbed" by Expose. I've tried relaunching Finder as someone here once suggested, but that didn't fix the issue. A reboot will fix the problem, but it will eventually resurface. Any ideas - I'm thinking a corrupted plist, but who knows.