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We seem to get more clicks than show up in logs, although this is just a feeling rather than actual counting.
(To say that I'm not going to discuss anything remotely like a 'detail' is an understatment.)
If you are interested you'll need access to about 5 machines and some scripting knowledge.
The results I found provide what I'd call 'good' protection from massive clickers, but in almost all cases (except VERY close clicks) the first two clicks would be charged against you. I can see why and, for most low-CPC keywords I don't have an issue. (User visits, clicks 'back' and then the ad again results in another charge.)
The only other comment that I will make is that a determined clicker (scripted) can cost you money, but nothing anywhere near the high-cost you would think.
In Short: I'm comfortable with the level of protection, and in the event that it is not sufficient such an incident would be, for most sites, extremely obvious, and I'm sure Google would happily credit you once presented with some logs.