News.com reports on a new Click Fraud Index study that shows the click fraud rate dropping from 12.8 percent to 11.9 percent in the 3rd quarter amongst top-tier PPC engines. Second-tier engines have realized an increase in click fraud with 23.2 percent from 20.3 percent. Overall, click fraud is down at 13.8 percent from 14.1 percent, quarter to quarter.
Postscript: Google asked me if they can make a statement about this, so here it is:
The estimates of attempted click fraud provided by this report are considerably more than the numbers we see on our network. More importantly, even if the numbers in the report could be believed despite the serious flaws we have previously demonstrated in their methodology, advertisers should understand that they include clicks Google has filtered and not charged for. As a result, the statitistics in the report do not actually measure the impact of click fraud on advertisers but the amount of fraud that may have been attempted. Google advertisers can see exactly the amount clicks we filter by running a simple report in AdWords