I have always been skeptical on the CPA model. They're been much hype and money around Google Adwords: People talking about Internet ads becoming relevant, non intrusive and what have you...
And now one discovers that all this may be the base for significant fraud and advertisers being charged way too much by Google.
Have also a look at John Battelle's account.
I am not an expert on this issue but a little voice tells me that this story is far from being over even though Google has already agreed to reimburse $90 million to advertisers.