I have observed this for some months now - Google displays the ad just above the top search result, with an eye-catching yellowish background, most-probably in an attempt to make the user click on the ad, instead of the top organic result. This type of behavior doesn't happen every time, probably because that would lead to:
- A deeper understanding of how AdWords works.
- Allegations of evil-practices by Google.
- Advertisers not wanting to buy ads for their own brand-name/product-name.
- The inevitable possibility of competitors copying this trick.
Quite likely, Yahoo did not intend this, but this is a fine example nevertheless: