The ad unit is a custom sized ad unit, but instead of a normal static background, these ad units have an animated arrow in the background, drawing a visitor's attention to the ads.
Here is screenshots of how they cycle through with the animated arrow pointing out the ads.
ThreadWatch points out these are "Made by Barry" of SERoundtable, using this image:
There are custom style ad units available to premium publishers and on a very limited case-by-case basis for regular publishers. But since all publishers must receive permission and approval from AdSense for each custom ad style before they put them live on a site, this means that this animated background ad unit style must have received the AdSense stamp of approval.
AdSense has done some testing with images in the background of ad units before, primarily special edition single day runs around holidays, such as the Hallowe'en themed ads and the US Thanksgiving ads. But I have never seen animated background images in ad units before.
What do I think of them? Personally, I am not a fan of this. As an advertiser, I would worry that it would unfairly draw attention to the ads and result in an increase in clicks from that particular site. Depending on the site, I would consider going as far as blocking my ads from appearing on publisher sites that would use this style of ad unit. I suspect publishers would be unhappy to see such unfair attention drawn to ads... after all, it is against the terms to have an animated arrow next to the ads pointing to them.
One of the ads I saw happened to be for SEOBook.com, so I asked Aaron Wall how he felt about his ads appearing in this style of ad unit:
I don't like the idea of click incentivizing on ads that I pay for by the click. It can increase ad spend and lower ROI. I don't think it is fair to charge advertisers by the click and then use various techniques that encourage people to click on them.
Yes, many people are using various techniques to entice clicks, both tricks that are against the AdSense terms and techniques I think should be disallowed. I would prefer if my ads did not appear on pages that did.
While I did love the themed ad units with images in the background, I do not like animated gifs in the background of ad units. And to keep advertisers opted into the content network, I hope this ad unit style gets pulled quickly by AdSense, even though it did get initially approved.
I do think custom ad units are a good thing for premium publishers and the high end regular publishers that need to utilize custom ad units for specific placement issues. But I think some of the flexibility allowed with custom ad units needs to be trimmed back a bit if publishers are receiving permission to use these kinds of click enticing background images in their custom ad units. If not, it could end up affecting more publishers than just those using it.