Derek Sivers spent two years trying to make Rails do something it wasn't meant to do, then realized his old abandoned language (PHP) would do just fine if approached with my new Rails-gained wisdom.
This O'Reilly Network article has two parts: the first is an overview of P3P, written by Simson Garfinkle; the second section, written by Lorrie Cranor, offers a more in depth look and examples.