I'm interested in sensing distance to user similar to the SparkFun CEO's Nike footpod car unlock project. ( https://www.sparkfun.com/tutorials/135 )
The project is to design an interface that unlocks the car as you approach.
I considered at first using Bluetooth, but the two modules I've ordered (HC-05 and BlueSmirf) have had issues. When I tried to get RSSI using inquire mode it only worked when the other party is set discoverable, which is intended.
I'm trying to get something I can detect the distance of... I have an Android programming background, so in theory, I could see if it's possible to initiate bluetooth connection (assuming already paired to bluetooth module). I could code something like that if it's possible in Java.
I considered the Nike, but I haven't figured out if you can get RSSI/signal strength out of this.
With wifi, I am wondering if you can get rssi of clients without being connected. Woudl be a pain to initiate a peer to peer adhoc wifi network between arduino and phone just to sense distance...
>:( 
So anyway, my question is as follows:
What is best for sensing distance of a user... I was thinking cellular phone, but am not opposed to the Nike+ Footpod: Wifi? Bluetooth? Nike+?
Your feedback is most desperately needed.