An interesting article in MarketWatch this week - they reckon that the vast majority of marketing budget holders not only acknowledge click fraud, but are unconcerned by it. This follows Google's news last month that they did not charge advertisers for the clicks that they knew were placed fraudulently. But even the ones slipping through the net don't seem to concern advertisers.
As one such man said, "A certain amount of mail goes missing - are you not going to write letters? Airlines occasionally crash, are you going to stop flying?". The general feeling seems to be that as long as the tactic is still making money overall, the percentage of click fraud (estimated to about 20%) is negligable enough to be seen as a business expense. And in fact, once you recognise it, steps can be taken to minimise the risk, as well as collect data on it. The benefit of having your advert read by someone who has been preselected as having an interest in the area is too valuable to discard.