They're also inside your cereal box. Check your pantry!

You spew data the entire time you are online. What do you care if its attached to a one in a million account, or some anonymous cookie? What is anonymity if it isn't being just an insignificant number in a much larger grand total? Your ideal of "protecting" your data as if its something important or interesting is very 1990's. Technophobic thinking like this is only in vogue with a select subset of Slashdot members and other minorities with ill-conceived notions about their online data.

In fact, why hasn't anybody already implemented a system like this?

You ignored google's options for this. If you have a google account, you can configure your preferences so that ads are better targetted.

I present to you, exhibit A: Google Ad Preferences

Mod parent and grandparent down. They have no understanding of life behind Websense or similar filters that receive automatic updates. They block anything easily found on Google and you'd be wasting your time. That is the core of the Websense business model - find and block content quickly and efficiently - and this is the exact circumvention focus of users on Tor, Haselton-proxies-r-us, etc.

robots.txt - well behaved robot spiders like Google's adhere to it. If configured properly, this would keep Google from indexing the site.