Google announced a change to AdWords ad spots earlier this week:
Starting today, ads that have previously shown to the side of the results may in some cases appear below them. We dynamically optimize each search page, including its ads, to provide the best experience for our users.
At the bottom of the SERP. Below the fold. It got a mixed reaction from offline agency people and online marketing professionals, with varying levels of outrage and knowledge. This change in real terms creates a greater difference between the top positions and those that would normally be off to the site in visibility. I am sure that this will be obvious in the click through rate as well.