BTProx is an open source application for Windows that will automatically lock your Windows Desktop if your bluetooth-enabled cellphone is no longer nearby,
It locks on one of your Bluetooth devices which are paired to the the machine. When you walk away from the computer the device is disconnected and the computer is locked after timeout. It is possible to run a single application together with the lock. This application may be your favorite script dismounting network drives, erasing those dirty files from the desktop or anything else. The program sits in Windows tray showing its current status with tray icon and tooltip baloons.
That is an awesome idea. Hopefully someone will come up with something like this for OSX and Linux as well.
Update: BlueProximity for Linux accomplishes the same thing in Linux.
It’s been on linux for ages.