A while back, Quixote had mentioned an idea of using this functionality almost like a RFID proximity sensor...Thread at Promixis. If this could be done, it would definitely be a cool HA use for your phone.
if you take a look at the site www.bluejackq.com , it has info on bluejacking with phones, PDAs, and even PCs. My idea is that if you could do a constant scan for devices like with the software "meeting point" that they have a link to, then you could set Girder up to detect when you are in or out of range, thereby telling whether or not you are home, or automatically starting a macro to adjust lights, turn on the tv etc., as soon as you reach the stairs of your home. Hypothetically you could even have it remember the name that it finds and greet the person if they are not recognized (like if you have a friend over and he has a bluejackable phone, it could say "welcome and make yourself comfortable, James!" That would really freak some people out.