For people who have iSight, or MacBook Pro with built-in iSight camera – you could use the camera to take snapshot for any invalid login attempts in your system:
With the new MacBook Pro's including built-in iSight cameras, I thought it would be useful to take an iSight snapshot of anyone trying to break into my laptop. This is done by tailing /var/log/asl.log for invalid login attempts (both at the login screen and screen saver). I have created an authsight program (1.5KB download) that provides a monitoring daemon which calls the camera after an invalid login attempt, and also includes a launchd manifest. An iSight command line tool named iSightCapture is also required.
Of course it wouldn't be very useful if it is already stolen, but you can track if your little brother or sister tried to access your Mac.
Download authsight program – [MacOSXhints.com]