So ever since I got Mountain Lion, I've been having this issue. When I first start up my machine, my hot corners (set to exposé, show desktop and dashboard) work. Then, inevitably, a few hours later they just stop working. The F3 button on my MBP (unibody core2duo mid-'09) doesn't trigger the exposé and my four-finger trackpad swipes don't work either. Killing the windowserver process, logging out/in or restarting fixes the issue, but each time I fix it I know it will just occur again. Sometimes it occurs within an hour, sometimes it takes a day, but it happens every time. My hot corners + expose + trackpad gestures are all configured correctly in prefs. Ideas?