Michael Grothaus from TUAW on the leak from the OS X Lion Developer Preview that Safari may include Do-not-track features in 10.7:
Do-not-track features in a web browser allow that browser to send messages to websites and advertising networks informing them that the user wishes not to be tracked. However, the feature is only effective if the advertiser or website agrees to not track a specific user, and right now advertisers and web sites are not legally required to honor the wishes of users using do-not-track features in browsers.
Grothaus nailed it. These features are great, but until it’s enforceable a la the Do Not Call List it’s really meaningless because advertisers will just ignore it.