I'd like to say if you report something once, it may be a fluke. If you report twice, then there may be something to it. But reporting something three times, well - to me, it is here to stay (at least for a while. This is the third time I am reporting a thread on folks spotting Google Checkout icons on Google AdSense ads.
I have still not seen them myself live on a Google AdSense site but it appears more and more people are noticing it on a daily basis.
Google Checkout icons can be seen on several AdWords ads on the top and right side of a search on sporting goods.
Scratch that, I have seen a Google Checkout icon on topix's sporting goods section. Scroll down a bit, and you may see an ad for "Dicks Sporting Goods." They accept Google Checkout and thus have the Google Checkout icon on the AdSense ad.
I think this just may stick for the near future.
Forum discussion at DigitalPoint Forums.