I've done it before and I'm doing it again. The Gang at ADNSF has really pulled through with Google Checkout integration on ML 8. I had my doubts at first, but we've processed more than 130 orders in 3 days using "GC" and all I can say is - "easy". After you set your preferences, Google Checkout automatically talks with the storefront, keeping you informed.
*Orders are created both on "GC"s server and your storefront, and look identical to regular orders with a "Google Checkout" payment type.
*If a customer cancels an order during their review phase of the order, the order status changes on your storefront, AWSOME!
*"GC" order charges can be controlled directly from the ADNSF Administrative Console, AMAZING!
In fact, I don't think there are very many, if any, other storefronts that have that level of integration.
Now let's say you don't want to use ADNSF's Admin side to process orders (like us, we just handle so many orders), well the good fellas at ADNSF have built a procise and easy to use API that will talk with "GC" if you need to integrate it into your cart/cart software, again AMAZING!
Something that the guys at ADNSF might want to do on their website is show the features that they've built for "GC", not simply that they've implamented (that's all I could find anyway) it.
But like I said guys, you deserve the praise for building such a well integrated product.
K-BL