I thought that above-the-fold still has to do with first impressions. If I get to a website and I see a link that I looks a mostly-like-what-I-want, but a link that is definitely-what-I-want is further down the page, I'll probably click the mostly-like-what-I-want link because I saw it first and it looks "good enough."
Let's do a thought experiment. If you put the ads all at the top of the page, and all the non-ad search results below the fold you'd probably get a reasonable click rate on the ads. Now, let's think about if we put all the non-ad search results above the fold, and all the ads down at the bottom. Do you think people would get to those ads and read them nearly as much?