GPS-enabled phones are great for getting around outdoors but not nearly accurate enough to be used inside buildings, even a cavernous convention centre, hospital or airport. WiFiSLAM acts like an indoor GPS, using nearby Wi-Fi network "fingerprints".
WiFiSLAM measures the signal strength and unique ID's from the wireless networks around it, then matches the signal strength "fingerprint" against a reference to determine the device's location. The location and your direction are then refined using the phone's on-board compass and accelerometer. This allows for a highly accurate measurement of your location to within a "couple of steps," according to WiFiSLAM.
The downside to this system is that the reference data needs to be collected beforehand. Someone has to walk around every room in the building a few times with a special application running to gather this data. [WiFiSLAM via Technology Review]
Image credit: WiFiSLAM