The Spy in Your Pocket

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The companies selling those services insist that they care about privacy. AirSage, for example, gets data from wireless carriers to monitor drivers' cell-phone signals and map them over road grids. That lets it see exactly where gridlock is forming and quickly alert drivers to delays and alternative routes. The data it gets from carriers are aggregated from many users and scrambled, so no one can track an individual phone. "No official can use [the data] to give someone a speeding ticket," says Cy Smith, CEO of AirSage.

Privacy advocates say that even with those safeguards, consumers should have a choice about how their information is used. Even anonymous data could, for example, reveal where a large group of people is headed for a protest. "These programs start out with the best intentions, but they expand," says Barry Steinhardt, director of the Technology and Liberty Program at the A.C.L.U. Some responsibility, of course, rests with the individual. Since his data were revealed, Clark took his mobile number off his business cards. Wireless carriers also recommend that customers avoid giving out their mobile numbers online. But Clark insists that the law should change to protect our privacy, no matter how much technology allows us to connect. "One thing we value in this country," he says, "is the freedom to be left alone."