Scott Barlow sent me a pointer to this story about using cell phone triangulation data to determine traffic congestion. The Bangalore Traffic Information System is offline at the moment--Slashdot effect, I'm sure.
Of course, you can tell more than how congested the roadways are. You can also determine commuting patterns--in aggregate--over time. When they redid I-15 through Salt Lake before the Olympics, all kinds of sensors were embedded in the roads to give UDOT this kind of information. This is a novel use of data that's already there to accomplish the same thing without the additional infrastructure.