An article over at MediaPost named Pay-Per-Trick: Half Of All Ad Clicks Deemed Fraud reads, "According to research released by Web analytics provider Clicklab, fraudulent clicks can account for more than 50 percent of all advertising fees attributable to certain categories." Can you believe that? You hear of companies paying people in India to sit by a computer and click on ads, but 50% of all clicks are fraud, WOW.
Forum discussion on this topic at Search Engine Watch. Other threads of interest include the click fraud software that helps you detect fraudulent clicks. Some of the software you can purchase to help you with that include; Who's Clicking Who and Click Auditor.
In addition, in the past archives you can find my notes on the Auditing Paid Listings & Click-fraud Issues session at the past San Jose SES show.