My Idea: Use the aggregate (and anonymous) cell phone activity on cell-phone towers to track traffic congestion along major traffic routes. Measure traffic speed by signal strength and tower-hops, measure congestion by number of phones on the towers.
Of course this is one of those ideas that are a hell of a lot easier said than done, and the MIT guys appear to have actually figured out how to do it. It's also a lot easier to do in Europe where everyone is on a single GSM network, whereas here in the US, there are (at least) 2 network protocols and several carriers' whose data would have to be aggregated. that said, I believe this could be a valuable differentiating service that each major network could offer to its customers or sell to the media at large.