Click fraud problem affects Google's AdSense just as others PPC concurents.

It seems that many peoples often forget that Goolge's AdSense as just immune to the click fraud problem as other PPC engines. Damn Spam! has posted an english comment on a click fraud incident (deutsch). You can change `s/AdSense/some other ppc program/g' all over the place - probability that this may happen is not any different. AdSense is just biggest and most known and so it's reports are more visible.

I should note that this (common immunity to click fraud) is not only apply to incidents that have occurred (registered) but also to many imaginable exploitations.
E.g.

1. Imagine person wants to get cheaper price/bid (as clicks buyer) than a real market price.
2. She/he starts click fraud campaign against that bid.
3. Convertion rate for other/fair buyers of that bid goes down (false clicks will never become buyers, they need to be alive for that).
4. Bid starts to fail. This is exactly what person expects.

All PPCs (and AdSense is not any different in this respect) are possible victims of such attacks...