Scott Johnson has some thoughts counter to Google’s position on click fraud that its “auction-based pay-per-click advertising model is inherently self-correcting”. It’s worth a read if you buy ads from Google or have revenue from PPC ads on your site because click fraud will affect you eventually if it doesn’t already: http://fuzzyblog.com/archives/2006/07/10/click-fraud-is-not-self-correcting/.
(I know Scott’s post is two months old but I don’t think I’ve linked to it before and I happened to re-read it today).