ClickSentinel has an interesting philosophy on becoming an open source community. One point I can strongly relate to is the Byzantine nature of click fraud. In other words, you often encounter situations where both parties claim they are right and discussion essentially stops. Neither party (in theory) has much incentive to trust the other as their goals are in opposition (the advertiser wishes to minimize the ad spend, while the publisher and/or ad broker wants to maximize the revenue earned from ads. This reminds me of discussions I used to have at AV where I noted that just because we (business ops) made claims about a customer's traffic, the customer has no real reason to believe or trust us. What's in anyone's logs does not constitute "proof" of anything; it is (at best) an indication of some activity performed at a web server, but could be subject to errors or even falsified.
I also find it interesting that Robert X. Cringely of PBS has noted the Byzantine nature of the click fraud problem in a recent article he wrote criticizing Google's customer service reps for a general lack of cooperation with their customers. I don't think this aspect of the problem is very well understood, and it doesn't help the process much that Google is unwilling (for now) to shed much light on how they determine what is and isn't fraudulent.