The problem with Google Checkout compared to PayPal is that only people with a Checkout account can buy from places that require Checkout. It's not the job of a vendor to get their customers to sign up with Checkout. It's hard enough to get the to agree to buy something. That extra hurdle is just too much.
When you buy from Apple, you buy using Apple's checkout process. Why ever would anyone think that Android would use anything but Google's checkout process?
iPads and iPods require the use of iTunes which pretty much requires an account. You have to do this in order to use the devices. Apple pretty much has a system that once you are using their devices you've got an account. So when a user is searching for apps they've already got an account. There is no extra hurdle at that stage.
