Clash of Internet Privacy Policies
Internet Explorer 6.0, which was released late last year and has gradually become the most widely used browser software, aims to discourage third-party tracking abuses. It does so by blocking such cookies unless the third party's privacy policy meets strict standards, like agreeing not to collect personally-identifiable information on the user it is tracking. These third parties must put that policy into a machine-readable format specified by the World Wide Web Consortium's Platform for Privacy Preferences, otherwise known as P3P.

The problem for the Web sites is that so far only 25 of the 100 most highly visited Web sites have put their own privacy policies into that format. And even fewer have worked with their business partners to ensure that they, too, have digitized privacy policies and that they conform with Explorer's rules. As a result, even when a Web publisher does business with a partner with a strict privacy policy, if the partner has not yet digitized that policy to comply with P3P and Explorer, the partner cannot place cookies on a user's computer.
http://www.nytimes.com/2002/10/14/technology/14ECOM.html?pagewanted=all&position=top