"Checkout" this button!

I'm starting to gather some information on how effective the new Google Checkout buttons have been in increasing click through rates for SEM (Search Engine Marketing). I have to imagine that the ads are gaining some lift from the rather large buttons that appear just below the paid search results. It's also interesting to see what advantage ads farther down the results gain from using Google Checkout as a payment method. In the game of search engine marketing, every advantage counts.