Researching into something for Google Analytics today, I came across their terms and conditions.

Did you know that by agreeing to these terms and conditions you agree to have a privacy policy on your website?

In section 7 of their terms and conditions, it states: "You must post a privacy policy and that policy must provide notice of your use of a cookie that collects anonymous traffic data."

For a link to the full policy: http://www.google.com/analytics/tos.html

This is also required with other Analytics packages such as Omniture, but as John points out, they provide a method for users to opt out of the cookie to prevent their information from being tracked. Something Google does not seem to be providing yet.