The three major search engines are cooperating to come up with a common defination of Click Fraud, according to John Battelle.  Slashdot also has a writeup (see below)

"The fact that these three bitter rivals can team up shows just how serious the industry has become about preserving the current online advertising boom that is currently underway. From the article: "Click fraud has attracted an increasing amount of attention amid class-action lawsuits and industry studies asserting advertisers have been collectively overcharged by more than $1 billion for bogus sales leads during the past four years. Google and Yahoo contend that those estimates are gross exaggerations generated by opportunistic lawyers and online advertising consultants hoping to cash in on the anxieties triggered by their calculations"

"Part of the difficulty in fighting click fraud so far has stemmed from the inability to come up with a precise definition of the practice, a problem the alliance hopes to solve.".

I just filed a petition for one of my clients last night in the 90 million dollars set aside for Click Fraud settlement; interesting that there’s actually no standard defination of Click Fraud.