A WebmasterWorld member said he saw Google trying out a special type of Google AdWords listing.
He said he was using the AdWords Preview Tool and did a search on something. He then saw that under an ad, which showcased a Google Checkout icon, was a link with a plus sign that read "Products from this Advertiser."
He said, that if you clicked on the link or the plus sign, the box would expand to showed 3 products, descriptions, and links to the merchant's site.
I tried to reproduce this action, but I was unable to see this behavior. It would not surprise me if Google was testing this feature for Google Checkout / Google Products merchants.
Forum discussion at WebmasterWorld.