We use adsense on some of our sites and use the allowed sites tool option. The tool will report sites that are showing your adsense that have not been approved. Typically they are just cached pages on the various search engines, but we found a couple interesting ones from yahoo and wondered if anyone has any insight:
It looks like this could be some sort of human "judging" going on at Yahoo? We have just started seeing this in the past month (newish site) and have seen an increase in Yahoo traffic during this same time (up many times fold). I believe that we may not always see a referrer in our logs because they are hosting our content on their servers (as is reported by adsense) and we would only see it in the logs if they clicked a link.