There are reports at both DigitalPoint Forums and WebmasterWorld that Google AdSense is testing a new ad format. The new ad format has "featured ads" within the ad units, with little yellow stars next to them.
I was to hunted down an ad myself to confirm it is real. I spotted the ad on ukcarinsurances.blogspot.com, here is a screen capture:
The ad, as you can see, has a yellow star next to it and if you mouse over the star, it using a title attribute that reads "Featured Ad."
I was only able to replicate this once...
There has been no word from Google on this yet, I will do my best to get an official statement shortly.
Update: Google sent me a statement confirming this test. They said:
We are currently running a limited test in which a small number of users are seeing ads that are marked based on signals related to quality and relevance. This experiment is part of our ongoing efforts to help users find what they're looking for, and we're closely monitoring feedback.