Andrew Goodman spotted Yahoo placing little shopping cart icons in some of the sponsored listings for eBay ads. Just like how Google has implemented it for those AdWords advertisers who have signed up with Google Checkout, Yahoo seems to be displaying those icons near the ads. I don't have any more details at the time, but you can see a screen capture at Traffick.com.
Postscript: Search Engine Journal has an update on this, where a Yahoo representative has confirmed that they "are testing various PayPal icons in some of our Yahoo.com sponsored search results. These icons are designed to help buyers identify where they can use PayPal to make purchases.” The spokesperson continued, "We are currently evaluating how users respond to icons in our sponsored search listings to determine whether such icons improve the user experience and help them identify merchant capabilities that are important to them.”