Gord Hotchkiss posts some interesting stuff about SEO on MediaPosts's Search Insider enewsletter.

The article below describes how he wanted to explore the way people respond to online search results, back in 2003.

Bottom line: organic beats paid, 70% to 30%.  And that's held up for the past 6 years.

http://www.mediapost.com/publications/?fa=Articles.showArticle&art_aid=111591