Proximity is a simple micro-app for Mac OS X that monitors your Bluetooth connections so it can tell when a wireless device is near by or not. When it detects one of your gadgets entering or leaving the vicinity, it runs user-defined scripts.
To get it to work, you just sync a particular Bluetooth device with your Mac, then select that device in Proximity’s preferences. You can then assign ApleScripts for both in range and out of range. This is really useful if you have a phone that syncs or tethers via Bluetooth, although there are certainly other good uses for the app. Since you choose the AppleScript, you can pretty much make it do whatever you want.