Essentially, I copied pieces from the various checkout_*.php steps, so that the "View Cart" page displays all the data on the order summary on checkout_payment.php, albeit with the ability to add and subtract items from the cart. So yes, this total includes all attributes and shipping. Although it does not display tax in this summary (because it doesn't know where you live), in the background it loads your tax zones and rates and keeps them in the XML sent to Google if the user checks out. The tax part works only for U.S. states.
My current version calls out to $shipping->cheapest() to get the cheapest of all the shipping modules you've enabled (this is the method labelled "Best Way" in the standard osCommerce checkout_shipping.php). That is just my personal hack because I have two shipping modules from which the user will always prefer the cheaper.
One thing I would like to do before packaging up this module is to add an Ajax shipping selection on the "View Cart" page, so that your user is allowed to select a shipping method before proceeding to Google, rather than just choosing the cheapest. This part will probably take me another few days as I know zero Ajax at the moment and Ajax is necessary since the XML will have to be repopulated after the shipping choice.
The final step is the order and customer processing. Behind the scenes it adds rows to customers and orders once Google calls home via the callback API.
If you want to get an idea of what's happening, and I'm embarassed to give out the link because you'll see that my site looks stock to the point of absurdity (my gf is working on a new template), check out http://www.officialclassicgames.com. Add an item to your cart, and then click on either "View Cart" or "Checkout." If you proceed to Google, you need to log in to your Google account before it actually populates the shipping and tax costs. But rest assured those came straight from my shipping modules and tax zones.
The only real disconnect of my hack to the osCommerce world is that I'm short-circuiting the concept of registered customers. My customers no longer log in -- they simply fill up their cart and then proceed to Google. Tell me what you guys think. I know it's a bit radical to eliminate logins -- what do you guys think? Also, my hacks do make Google the only means of payment.