"Based on my experience at AltaVista it is fairly easy to detect and disregard click fraud. You look for things like frequency of clicks per minute, number of clicks coming from the same IP address, seconds spent on the site, etc. This also explains why the Tier 1 sites have lower click fraud than the Tier 2 and 3 sites, The Tier 1 sites are more sophisticated and spend more money on click fraud detection and prevention."
This is good as far as it goes, but it doesn't take into account the more deceptive types of click fraud that occur, such as automated clickers that are distributed via malware and operate, for all practical purposes, as if they were humans generating ordinary traffic. Another thing to keep in mind is that while he was at AV, AV was a very popular Internet destination; not just among search engines but among all Internet sites. So the amount of (actual) click fraud would most likely have been masked (from the advertiser's standpoint) by the amount of conversions (sales) the advertisers got. Whereas after he left, AV lost lots of market share, and its traffic numbers dropped considerably. As the purchasing customers left for Google, Yahoo, etc., the amount of "traffic of good intent" dropped, exposing the fraudulent traffic. In effect, AV was more like a Tier 3 site than a Tier 1 site, from the advertiser's perspective.
I remember hearing his name and seeing it in emails, but I don't think I ever met him. Elsewhere in his blog, he gives some personal perspective on why AV failed.