Google is trying hard to get us to use the term "invalid clicks" rather than "click fraud." I doubt that will happen, but this is Google we're talking about...anyway, it's the weekend, so I haven't had time to read them, but they reference a couple very documents that sound very interesting, especially the 47-page report by Dr. Alexander Tuzhilin from NYU.
I'm interested to see how much Dr. Tuzhilin's report reveals. I think click fraud is a much bigger problem than Google lets on publicly, but I also think Google does the best job of filtering out those useless clicks and being proactive about refunding advertisers' accounts for fraudulent (invalid) clicks. As an advertiser I like to know what Google's doing to catch the fraud, but I also worry that it might give fraudsters too much information and ideas for ways to beat the system (which is why Google has been so close lipped in the past).