There's been a couple of threads on WebmasterWorld concerning irreconcilable (not necessarily fraudulent) clicks. One of the thread participants pointed to iMediaConnections web analytics area which contains several articles concerning the limitations of accuracy of pageviews, unique users, clicks, etc. I've read a few, and most of them agree with what I've been saying about web analytics in general.

Another thread asked if Y! would charge for clicks from partners if the advertiser blocked traffic (based on IP addresses) from said partners that seemed suspicious. Theoretically, Y! does not know that the IP addresses are being blocked, because it is not involved after the click on the ad redirects the browser of whoever made the click to the advertiser's site. This was an issue of concern at AV and another reason why I think PPC advertising is a bad business model.