I implemented system idle timer according to the advice given here some months ago (http://www.cocoabuilder.com/archive/message/cocoa/2004/10/27/120354), but unfortunately this piece of code crashes on Tiger. I also tried to use CGSSecondsSinceLastInputEvent, as pointed out by Evan, but it gives me linker error: "undefined symbols CGSSecondsSinceLastInputEvent". I inserted the prototype and tried to link ApplicationServices framework but still can't get rid of this error. Is there something obvious I'm missing?