I came across an interesting paper on P3P (P3P is the Platform for Privacy Preferences Project) which is a W3C standard for expressing the privacy policies of a web site. The paper is from the CMU CyLab, and can be found here (PDF format).
A primary user of P3P is the Internet Explorer browser. It uses a "short form" of the policy to make decisions about whether a web site meets the security criteria one may set in the browser. Since most people never bother to configure different security levels for different sites, in practice any P3P descriptions that match "Medium security" will pass the security check.
The brief summary of the paper is that many of the top sites do not use P3P. Or, if they do use it, they make mistakes in the policy which will confuse browsers. And worse still, some sites seem to use P3P to actively trick browsers into thinking the site gathers no private information when it in fact does.
The paper is long, but many pages are part of an appendix. The main section of the paper is relatively short, well written, and is an interesting read.