An application from Foozoo Design called Lockdown will monitor your computer while you're away and sound an alarm if someone tries to steal or otherwise access your system.
Lockdown is based on the open source project iAlertU, but the new version features a significant interface overhaul. The application has the look and feel of a modern Apple application. Lockdown also contains numerous minor bug fixes.
Lockdown uses the computer’s motion-sensors, keyboard-sensors, external device sensors and MagSafe sensors to monitor the computer. When the alarm is set off, the application prevents the sound from being muted and then takes a snapshot of the offender using the iSight camera.
You can also use Apple’s remote to arm and disarm Lockdown.
Lockdown 1.0.4 is a free application and requires a Mac with a built-in iSight and Mac OS X Leopard.