The latest numbers from Chitika.com seem to confirm the obvious. Dumb people click ads. Smarties don't.
This is also why in general MSN, Ask.com and other ads tend to yield much better clickthrough rate and ROI for advertisers. Unfortunately, you just can't get enough search volume there to put enough money in.
Funniest example I've heard lately: Wordpress shows ads to users who are on Internet Explorer, but not to anyone else. That sounds like a hell of an intelligent hack for content providers.