Google had previously announced about this +1 button on Adsense ads to publicly promote the ad. It was said to go live in October 2011. But we are seeing it live on almost all the rich image ads these days. Maybe from Christmas.
It’ll show the faces of people who liked it, only if they happen to be your friends(on your list: GMail, Circle contact).
Here is a live video of the ads with +1 button, which we could spot on our blog:
You can read more about this here: Google’s +1 On Adsense Ads: Demo Video!