9/12 Req: Danny Sullivan, "Buying Your Way In: Search Engine Advertising Chart"

http://www.searchenginewatch.com/webmasters/article.php/2167941

Do search engines sell listings? Yes. Should searchers fear this? Not necessarily. Paid listings generate revenue for search engines, which in turn helps them provide unpaid editorial listings to searchers for free.

Think newspapers. Newspapers have both "editorial" copy, which is not supposed to be influenced by advertising, as well as ads themselves. You may read the paper primarily for the articles, but there are certainly times when you may find the advertisements useful, as well.