Emma Connors and Mark Jones have written about click fraud in today's Australian Financial Review. Unfortunately the article cannot be viewed online for free. They write that Nielsen/NetRatings is starting research into this issue in Australia. They are going to look at the behaviour of IP addresses, exactly what I said in the same article!
Also quoted is ClickSentinel, which claims that 30% of clicks are fraudulent. I wonder if Shuman Ghosemajumder's team at Google will look into this claim next. :)