Proximity Sensing via Cell Phone

This story on describes how Jonas Salling, a Swedish programmer, developed Apple software to connect a Sony Ericsson cellphone to his Mac using Bluetooth. A lot of emphasis may be placed on the use of the phone as a remote control device for the Mac. This has also been down for Linux as well. However the one little fun thing that he did, which allow the laptop to detect the presense of the phone (and its owner) via bluetooth, is in my opinion, the innovation. Start thinking of application for this technique.