Earlier this year, a hacker by the name of Mehdi Lauters published some data from an experiment he conducted using two Raspberry Pi devices running GPS, WiFi, and Bluetooth sniffing applications. Put simply, he traveled the city of Bordeaux, France, and collected wireless data from signals within his range. After collecting this data for six months, he summarized and published his findings here.
Here's a summary from his write-up on the project:
You want to discover your city's public transport infrastructure? If people crossing your street are mainly tourists or neighbors? Check if you always take the tram with a given person who likes pizza and travels? Or maybe more when your neighbor is at home or not right now, and use to be there at this time?
Mehdi discovered 62 postmen using the Facteo app on a Samsung device, and he was able to determine which commuters on a particular train knew each other, based on their phone connections.