blueproximity -- Locks gnome desktop by detecting a bluetooth device's proximity
http://blueproximity.sourceforge.net/
This software helps you add a little more security to your desktop. It
does so by detecting one of your bluetooth devices, most likely your
mobile phone, and keeping track of its distance.
If you move away from your computer and the distance is above a certain
level (no measurement in meters is possible) for a given time, it
automatically locks your desktop (or starts any other shell command
you want). See end of this file for interesting commands.