I cannot remember the last time. It has been long ago. But I also did not see anything interesting where I have to click. Sure people follow their conversion rates. But then the money probably comes from people that are using the internet in adifferent way than I am.
I don't think ive ever clicked, and if i have i closed it really fast. there are clearly people that do it though, or else we would all be out of a business.
You have to remeber that for eveyone of us that know about adsense, there are 10 noobs that have no idea what adsense is and click on ads?