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Google is testing a new AdSense ad format with select publishers, commonly known as sticky ads. Google is calling them anchor/overlay ads and vignette ads.
The Anchor/overlay ads are ads that are mobile specific and they stick to the edge of the user's screen and are easy for the user to dismiss. The vignette ads are also mobile only and are full screen ads that appear between the page loads on your site and can also be dismissed by your users.
You may be able to see them on the page-level ads section of your AdSense console.
Some publishers have seen this option for the past week or so and are discussing it at WebmasterWorld. Jonathan Weber sent me a screen shot, here it is, note you can click on it to enlarge:
Forum discussion at WebmasterWorld.
