I think if you read what they have written “Google Checkout users click on AdWords ads 10% more when the ad displays the Checkout badge, and convert 40% more than shoppers that have not used Checkout before.”

Basically If I have previously registered as a GC user then I am 10% more likely to click on a PPC ad with the GC icon – which is as to be expected.

Regarding the 40% conversion claim it is clear that if I have registered as GC customer in the past then I am hugely more likely to convert when on a site that has the GC icon because I am a “member” of that payment process so to speak? I trust it and have used it in the past.

Agree 40% sounds a bit far fetched though.