As I’ve been saying…
To Aim Ads, Web Is Keeping Closer Eye on You, New York Times, March 11, 2008
…A new analysis of online consumer data shows that large Web companies are learning more about people than ever from what they search for and do on the Internet, gathering clues about the tastes and preferences of a typical user several hundred times a month.
These companies use that information to predict what content and advertisements people most likely want to see. They can charge steep prices for carefully tailored ads because of their high response rates.
The analysis, conducted for The New York Times by the research firm comScore, provides what advertising executives say is the first broad estimate of the amount of consumer data that is transmitted to Internet companies.
Privacy advocates have previously sounded alarms about the practices of Internet companies and provided vague estimates about the volume of data they collect, but they did not give comprehensive figures.
The Web companies are, in effect, taking the trail of crumbs people leave behind as they move around the Internet, and then analyzing them to anticipate people’s next steps. So anybody who searches for information on such disparate topics as iron supplements, airlines, hotels and soft drinks may see ads for those products and services later on.
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MichaelZimmer.org 
It will only get worse. For now, for those who want more privacy it’s probably good enough to block ad servers and third-party cookies, and clear cookies and change IP addresses regularly. But in reality, the combination of User Agent, monitor resolution, IP address range and a few other factors could be mined to pinpoint many users pretty well regardless of cookies. And with increased sharing of these digital fingerprints, companies (and governments) could regain the upper hand, forcing the die-hards to use more aggressive tactics like anonymizing proxies, varying User Agents, etc. The arms race continues…