A number of people have wanted a tool to detect BlueTooth devices and automatically do something when a signal is found or lost.
I just came across a little gizmo called BlueSense on MacUpdate:
Detect the presence of your Bluetooth device and execute actions on your Mac.
BlueSense analyzes the signal of your Bluetooth device and executes any desired actions when you move or come back at proximity of your Mac.
It's only $3.99, so some of you might be interested.
I'll probably take a closer look after I get my iPhone for Christmas.