TechCrunch recently just did an article on Facebook Click Fraud, it can be found here: http://www.techcrunch.com/2009/06/21/facebook-click-fraud-enraging-advertisers/. Thanks to the users in WickedFire for pointing this out.

It is really unfortunate, there is a terrible amount of click fraud all over the place. Even a year ago sometimes we saw up to 30% of traffic coming from some yahoo sources having click fraud in them. As new systems like FaceBook Ads and MySpace ads come out there is even more click fraud taking place. Many of theses new sources to buy traffic aren't yet sophiscated enough to detect click fraud. And unfortunately it isn't normally their top priority because they benefit from the extra clicks. Be aware that this exists, and unfortunately sometimes, it just something you have to factor into your cost. The article covers alittle bit on whats going on there.

http://www.techcrunch.com/2009/06/21/facebook-click-fraud-enraging-advertisers/