The Internet Architecture Board held a workshop last year to discuss the problem of unwanted traffic -- including spam, DDoS attacks, and even click fraud. I hadn't heard about this until yesterday while reading a post from Sean Donelan that mentioned click fraud. A few searches led me to the workshop proceedings. Interesting stuff. I sent email to SD and the authors of the draft report asking if there was going to be an IETF WG formed to tackle the unwanted traffic problem, and gave them some of my thoughts on click fraud (including pointing them to this journal). Incidentally, Lixia Zhang, one of the draft authors, was a doctoral student while I was an undergraduate. I remember seeing her at my research group's meetings and in the graduate version of 6.033. She also worked on TCP performance.