I just found a very nice post describing nice things to do with Bluetooth. By using the Proximity tool, my MacBook Pro can monitor the precense of my cell phone. Proximity will run an AppleScript when a selected device enters and/or leaves Bluetooth range. Most phones are class 2 Bluetooth devices, which gives a range of 10m (outdoors). This allows you to automatically lock your desktop when you leave and unlock when you come back.