I am developing a system to locate an iPhone indoors. I am considering using Bluetooth triangulation based on signal strength.
It's for a restaurant type app. The restaurant wants to know at which table the phone/app is sitting.
If your iOS device supports Bluetooth LE, you can easily obtain the RSSI between it and sensor stations, as well as a UUID for identifying the device. You can also read the MAC address if you're using the right software on your sensor side, but I don't believe that this information is exposed by Core Bluetooth in the same way that RSSI and UUIDs are.
There are two ways to approach triangulation if using Bluetooth LE. One is to do the triangulation by monitoring the iPhone from a series of placed sensor locations, and the other is to do triangulation on the iPhone itself by reading the signal strength between it and devices that are advertising from known points.
The latter can be done by placing a series of LE peripherals in known locations and having them advertise at regular intervals. Within the advertisement information, you could stamp the location of that peripheral in the room. The iPhone could read these advertisements, get the RSSI information from them using the -centralManager:didDiscoverPeripheral:advertisementData:RSSI: delegate method, and triangulate its location.
The former approach would be a little more involved. It would require that the iPhone itself be placed into an advertising mode using the new iOS 6.0 support for making the iPhone a Bluetooth LE peripheral. The sensor locations could then pick up the RSSI from them to the iPhone via these advertisement packets, as well as a UUID you generate for the phone. You can also pick up the MAC address of the phone is running the right software on the sensor nodes. A central server would then need to combine these readings to triangulate the iPhone's location.
You might need to experiment with the transmission strength to determine what kind of resolution this would provide. The RSSI information is noisy, and in my experience it only seems to resolve to +-30 feet at the normal transmission levels for my peripherals. You may be able to improve upon this with multiple sensor stations and a lower transmission strength.