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A Washington, D.C., man who is desperately trying to quit smoking receives a letter from a marketing firm: "Our records indicate that you have tried to stop smoking using a prescription nicotine replacement product. We hope you have succeeded, but if you, like many others who have tried to quit, are still smoking, we have good news for you." The rest of the letter touts a new cessation drug called Zyban.

Meanwhile, a week before her birthday, a Pennsylvania woman gets a card in the mail from RadioShack, wishing her happy returns and offering $10 off her next purchase at one of the company's retail stores. She has never bought anything from the store and never told RadioShack when she was born. No matter. RadioShack bought the information from the state's motor vehicles department.

Technology has changed the rules of privacy. We go through life inadvertently dropping crumbs of data about ourselves. Following right behind us are powerful vacuum cleaners--computers accessed by marketers, snoops, and even criminals--sucking up the crumbs, labeling them, and storing them for future reference.

"Privacy is like clean air," says Kevin Murray, who runs Murray Associates, a New Jersey­based firm that sweeps clients' offices for bugs and other surveillance equipment. "At one time there was plenty of it. Now it's almost gone."