Recently I played with Google Checkout a.k.a Google Wallet. My task was to make it work as option in payment methods step of Onepage Checkout.
My version of Mangento is CE 1.6.2.0
By default we have Google Checkout form only on the cart page, but not on Onepage Checkout page. I looked at the admin settings of the module ( Google Api ), but didn't find an option to enable it for the checkout page.
I found, that we have form template in googlecheckout/form.phtml and it seems, that it was planned to work on the checkout page.
I decided to "shoot" and created simple extension, that rewrites Mage_GoogleCheckout_Model_Payment so I changed
protected $_canUseCheckout = false; to protected $_canUseCheckout = true;
and voila, the option appeared on the checkout page as payment method.
I did some test orders in sandbox mode and everything seems to be fine, the products and the shipping amounts are correct. Also the billing information was automatically populated in Google Wallet page and it wasn't needed to populate again the same information of the checkout page again in Google Wallet.
What bothers me is the fact, that by default this "feature" wasn't available and may be there is a reason, why the folks of the Magento core made it work in that way.
Basically my question is do you see any risk to use Google Checkout as option in checkout page?
This is not allowed according to google policies.
http://checkout.google.com/seller/policies.html
'Do not position or present Google Checkout as a form of payment equivalent to Visa, MasterCard, etc. Google Checkout aggregates existing payment types to allow buyers and merchants to transact. Google Checkout is not a form of payment that replaces or is equivalent to existing payment types and must not be presented as such. Buyers using the e-commerce provider's standard checkout flow should not, therefore, encounter a payment field drop-down with Google Checkout listed alongside payment types like Visa, MasterCard, American Express, etc. Instead, the buyer should have the option of selecting Google Checkout as a distinct checkout flow as required in 4b.'