It appears that Google's new checkout system provides an added bonus and strong incentive - namely a nifty little icon that helps your site stand out in the AdWords listings on search results pages. See below:

Notice how thefurniture.com has that spiffy green cart next to it? My guess is those advertisers are getting CTRs like they're the #1 result, despite paying for #7 placement. With an offer like that, I'd drop Paypal, too.