The US government is developing a massive computer system that can collect huge amounts of data and, by linking far-flung information from blogs and e-mail to government records and intelligence reports, search for patterns of terrorist activity.

...

While privacy laws do place some restriction on government use of private data - such as medical records - they don't prevent intelligence agencies from buying information from commercial data collectors. Congress has done little so far to regulate the practice or even require basic notification from agencies, privacy experts say.

Indeed, even data that look anonymous aren't necessarily so. For example: With name and Social Security number stripped from their files, 87 percent of Americans can be identified simply by knowing their date of birth, gender, and five-digit Zip code, according to research by Latanya Sweeney, a data-privacy researcher at Carnegie Mellon University.

http://www.csmonitor.com/2006/0209/p01s02-uspo.html