I had this problem in Tiger on my PowerBook, and now in Leopard on my new iMac. In System Preferences, I have various mouse buttons set to activate Expose, but no quick keys set. This works fine, but once every couple of weeks, I find my mouse buttons don't do any Expose actions. If I open System Preferences, I find the Expose settings are all blank. The only similarity between this iMac and my old PowerBook is that I am using a Logitech MX518 mouse. I have no mouse software installed. Anybody else have this problem? Is it a case of trashing some prefs file to sort the problem?