Using bluetooth as a proximity sensor.

Uncle Google finally had some words of wisdom to impart, I suppose this might be the reason?

It's fun when I get to repeat myself.

1. The iPhone doesnt stay in discoverable mode unless initiated by the settings screen.
   
2. The iPhone doesn't support the serial profile, so your device won't automatically see your the >phone when it is in range.
   
   

If you can use a Bluetooth device that supports the HID profile, you might have better luck...

http://forum.arduino.cc/index.php?PHPSESSID=ltukrr9ec6v8gi0l4na82jhsu3&topic=100026.msg750645#msg750645