A DigitalPoint Forums thread reports one member seeing a Google Checkout icon on AdSense ads. Typically, you see a little shopping cart icon when you search at Google and the sponsored listings are from a site that accepts Google Checkout. I personally have never seen a live Google AdSense (ad on a publisher's site) that had this little shopping cart icon.
The DigitalPoint member took a screen capture as evidence (although the screen shot technically could have been doctored up, I doubt it was).
We first reported this behavior earlier this month with Is Google Testing Google Checkout Icons on AdSense Ads?
Forum discussion at DigitalPoint Forums.