freshmeat.net: Project details for BlueProximity
BlueProximity helps add a little more security to the desktop. It does so by detecting a certain Bluetooth device, most likely a mobile phone, and keeping track of its distance. If it is moved away from the computer and the distance is above a certain level (no measurement in meters is possible) for a given time, it automatically locks the desktop (or starts any other shell command wanted). Once away, the computer awaits its master’s return: if the device is nearer than a given level for a set time, the computer unlocks without any interaction (or starts any other shell command wanted).