Google announced last week that they are changing the position that ads will show on their search engine. Ads that use to appear above search results have been moved lateral to the right side and some will be moved to the bottom of the page, below search results. The company has been making changes to the appearance of their search engine for quite a while now as they announced last Spring that important layout and design changes will be taking effect.
Google stated they felt displaying the ads below search results provides a more consistent viewing experience on their search engine. They have provided a useful tool Top versus Side that will help evaluate the performance of ads placed at the top versus the side (and bottom) of search results.