It seems to me, if eBay believed in its payment service they would want to compete head to head. I think it is telling that Google offers links to competitive services. For example, if you search for an address on Google you get links to Google maps, Yahoo maps and MapQuest. If you believe you have the best service you look to create opportunities for customers to compare which positively reinforces a positive view of the best service. eBay is making a mistake by banning Google Checkout, head to head competition is the way to prevent the Google Checkout beach head. My guess is that right now eBay and Paypal provide a more robust feature set and that difference may be enough to slow or stunt Google Checkout. That attack should occur now not later after Google has improved on their service.
I once read an article where Meg Whitman talks about the value of the eBay community. It seems to me if the community wants to use Google Checkout then eBay should respect the communities demands.