Using phones to determine where groups are gathering
Here's one of many companies working on using phone location-based services.
This one happens to have been founded by some current and former MIT folks.
http://www.citysense.com/home.php
http://www.columbia.edu/cu/news/08/06/citysense.html
Software uses pattern recognition algorithms to monitor location data in
real time, apparently to provide data to marketers: