It can't be any worse than the regular click fraud on the content network. ;) I'd venture a guess it would be FAR less actually.
I would also bet G will use a model similar to what it does now. The more money an advertiser will bring to G and the publisher, the more likely his/her ads are to show. This would likely mitigate most attempts at fraud. The lower the conversion rate, the less the advertiser brings G and the publisher, so the MORE he would have to pay per action to be shown. (just a guess of course).