Today there’s confirmation that it is indeed a test which is being done on a minimal number of impressions of publishers ads. The following was the response from AdSense to one publisher asking about the new images in ads:
“Thanks for your email – as you’ve mentioned, the change that you saw was part of ongoing testing to improve our ad formats. Such variations are a normal part of our testing process and generally run for short periods of time only. No permanent changes are planned at this time.
To maintain the integrity of ongoing testing, we do not permit publishers to opt out. However, please rest assured that such variations should impact only a very small percentage of your impressions.
We appreciate your patience and understanding as we work to improve our program.
The Google AdSense Team ”