A WebmasterWorld thread has a picture (as displayed on this site) of Google supposedly placing images within the Google AdSense text ad blocks.
In any event, I am not sure if this is a legit test or some toolbar or extension warping the AdSense ads for this particular user. I am personally not seeing this or able to replicate it but Google is known for testing and testing these types of things.
Would you like to see Google expand this test more?
Forum discussion at WebmasterWorld.