Via Seth Godin, here’s an article on Search Engine Watch about how users evaluate the Google search results page. They conducted of (only) 50 users to see where they looked in the Google search results.
100% of them looked at the top three results, declining down to 20% looking at the tenth ranked item (the last one on the first page).
This sounds dubious to me, but apparently 50% of searchers look at the top-place ad on the right-hand side of the page. Speaking as a guy who, ironically, runs several of these campaigns for clients, I never, ever look at the ads. It’s probably because I trust Google’s algorithms a lot more than the potentially-ethically-compromised advertisers.