Alex Bell (MSEE'10) is featured in today's New York Times in an article discussing how several organizations are gleaning data from ubiquitous cell phones to gather information about urban services such as subway trains:
"Projects Use Phone Data to Track Public Services"
http://www.nytimes.com/2011/06/06/business/media/06transit.html
To infer when and where each subway train is along its route, the system looks for phones that lose connectivity to the cell system at one point (suggesting the phone's owner has gone underground), then reappear sometime later at a different point (when they regain the surface). Since the arrival of a busy New York subway will result in dozens of people emerging from the station, it will be feasible to reliably position the train once enough people are running the app. This information can then be displayed to users, as the incentive for installing the app.
Bell, who developed the idea while a Masters student in the EE department last year, is now commercializing his idea at Densebrain, a company responsible for several other smartphone apps aimed at helping people navigate in cities.