Pretty much everyone has a mobile phone. When you are doing ubicomp prototyping it is often difficult to do field tests on large systems because there is normally a shortage of devices you can use in a field trial, and/or it is difficult to get users to carry around and charge a secondary device. Bluetooth is an excellent method of approximating identity and proximity detection for ubicomp systems.

I have developed a really simple, fast, easy to use bluetooth scanning system, that users RSSI (Received Signal Strength Indicators) to determine Nearness/Far-ness, as well as perform Asynchronous name lookups, and find hidden bluetooth devices, for the use in prototyping systems that detect nearby users:

http://code.google.com/p/python-bluetooth-scanner/