So I was trying to redeem an awesome deal at Buy.com, and it involved the $10 discount with Google Checkout. So I get ready to check out, and there's no way to get to Google Checkout. I scoured the FAQ's and Google's site, and nothing! Finally, I noticed somewhere it said that Google Checkouts would be treated as guests. So I log out, find my stuff again, and there's the button, hiding below the little sidebar shopping basket. So I log back in, and sure enough, there it is.
So the moral of the story is: once you've added your last item, DON'T click the big "Proceed to Checkout" button. Click the Google button that may be hiding farther down. If you've already gotten all your stuff, add something and then remove it so you get the sidebar cart, because that's the only way I can get it to show up.