IIRC, Google Checkout also required US customers only. As it was never extended to the rest of the world, I expected it to never take hold and change the marketplace.
Don't think that is the case. It was limited to merchants in a few countries, but it was not limited to US customers. I once purchased a computer from UK using Google checkout, and I live in Norway.
Perhaps its only true for merchants - I live in Singapore and I could use Checkout to pay for Cadence and Slang.