europeforvisitors - 11:15 pm on Jul 20, 2006 (gmt 0)
I can't remember if this was referenced earlier (I'm not re-reading every page of a 17-page thread), but Webmaster World recently had a "featured home page discussion" titled "Study: Click fraud could threaten the pay-per-click model":
There are problems with the CPA model as well. It's difficult for us to measure the exact impact of click fraud, but 15% sounds like a pretty reasonable estimate. On the other hand, our affiliate program generates about 50% bogus leads.
14% [the study's estimated rate of click fraud] is the cost of doing business. If it stays at that rate, I don't see it as a major problem.