PPC experts Andy Beal and Andrew Goodman both comment on their blogs about click fraud. Essentially, they have a view that the click fraud meme is over-hyped, and that eventually people will get on with it, and accept it as a part of doing business. It won't "take down" Google.
I agree that is what will happen, but I think it will be a bumpy road. The reasons Google is more vulnerable than say Yahoo are:
In other words, managing outside perceptions is going to be important for dealing with Click Fraud. Outreach with advertisers and agencies, along with changing some of the ways the Google accounting system works will be important. A simple thing they could do is establish an independent auditing council.
In the modern history of advertising, 3rd-party auditing has been an accepted way to deal with the fuzziness of counting audiences. Even though PPC ads can be counted for in a pretty direct way, that doesn't mean that there isn't a role for auditing.
If Google wants to be proactive on click fraud, and minimize its risk to their business, they might consider independent auditing as one part of the solution.