DP is a large site and I am sure it gets tons of traffic each day, so how does it protect itself from click fraud. I am sure there are 100's of people owning webmaster forums that might want to get rid of the compitition by making fraudelent clicks.
I've been wondering that too.. I think Google pays extra attention when things like that happen to bigger sites. They won't disable Adsense for sites like this because it would lose them a lot of money and hurt publishers. I guess it are only little sites that have to worry about things like this because its easier for Adsense to ban those accounts. Not sure if that's what they do but it makes sense with all these people getting banned for invalid clicks even though they never clicker their own ads.
DP displays randomly it's members adsense ads, you have no way to fraudclick to make anyone banned, only thing can happen is to "benefit" most of it's users, because their ads have more chance to be clicked.
Id say with a forum like DP, that google will understand with its heavy traffic and popularity and deal accordingly, i would imagine that they look after the sites of this size, its in their best interests.