From:
http://blogs.discovermagazine.com/sciencenotfiction/2008/07/21/of-hackers-and-batmen/#more-133
The sensor network in The Dark Knight relied making regular cell phones emit and receive precisely timed supersonic signals. This sonar data from each cell phone gets sent to the Batcave, where it is combined to create a detailed, real-time, 3-D picture of the entirety of Gotham City.
While the idea that regular cell phones can be hacked to have such capabilities may be pure fantasy, the idea of networking sensors together to create powerful aggregates of data is not, even when those sensors are themselves relatively primitive and distributed haphazardly.
At the HOPE conference, hackers were testing this idea out in practice. Many attendee badges had a low-power radio transmitter circuit built right into the badge. This radio transmitter simply broadcast a serial number over and over. An array of receivers spread out throughout the floors of the Hotel Pennsylvania where the conference was being held picked up the badge’s signal. Each receiver was networked to a central processor. Using data from multiple receivers, the location of the badge could be triangulated. A web site was set up that allowed anyone to see where every badge was at anytime during the conference. Popular talks or vending tables were easily identified as badges clustered around particular locations. New ways to mine the data kept being added on throughout the conference by caffeinated coders, creating rich features from the complex aggregate of simple data.