A good post at seochat on both sides of the clcik fraud argument.
Basically the two sides are:
Certainly there are a lot of "whisper numbers" from the click fraud detection outfits. Generally these estimates seem inflated and embellished, without any hard proof. The commenter at seochat who quipped "95% of statistics are made up" is probably on to something. Also, launching lawsuits to garner attention isn't proof of accuracy.
Google and Yahoo, of course haven't shared any significant statistics or proof on click fraud either. So until they or some credible, non-conflicted 3rd party does (like a university researcher), Google and Yahoo can't really complain about the hyped reports.