I don’t know how much effective this, but the goal according to the author “is to shut off the screen immediately after you leave, even if you just moved the mouse 10 seconds ago”.
It uses ultrasonic sound on your computer's regular sound system. But unfortunately not all Laptops are able to detect ultrasonic sound properly, but many could.
This new user presence detection technique may be more reliable than the slow and error-prone mouse and keyboard activity timeouts.
The open-source software works on Windows7, Vista, XP and Linux. Interested users can download it form author’s site.