I’m working on a project using Bluetooth to build location aware apps. The new Bluetooth 4.0 standard is a lot more power efficient, and more importantly, allows for unpaired data advertising. This means any number of Bluetooth devices could communicate to phones without first having to go through the tedious manual process of pairing.
I wrote a simple proof-of-concept program in Objective C, which listens for the Bluetooth transmitters and displays which is closest. It works by comparing the signal strength (RSSI value) of each device.