The new annotations appear when a user hovers over the +1 button. Hovering over it will display a list of friends and contacts that have already clicked the +1 button for that page. Google has also unveiled new code for the +1 button that will display the faces and names of friends that have used the +1 button. This feature works much like how the Facebook Like button appears for Mashable stories, displaying how many people have +1′d the page and which friends have +1′d it.

via Google Adds Friend Annotations to the +1 Button.