europeforvisitors - 8:28 pm on Jul 4, 2006 (gmt 0)

Well, we have only the SEs'/ad networks' word that they are able to recognize what they choose to define as invalid clicks. (Note that this might actually differ from what the advertisers are willing to pay for, which is the ultimate issue at hand here...)

Yes, and the ultimate test is whether advertisers trust the SEs/ad networks to deliver what they've promised (or at least to deliver an acceptable ROI). To judge from the numbers in Google's quarterly earnings reports, advertisers aren't dissatisfied with the network or its pricing model. (Which isn't to say that Google might not offer CPA for advertisers who prefer that pricing method, just as it offers site-targeted CPM ads for advertisers who prefer traditional run-of-site CPM campaigns.)