Google checkout has a long road ahead of it before it becomes much of anything.
One of the best ways to keep that road as long and winding as possible is to limit its exposure.
Ebay would quickly give Google the kind of exposure that could lead to its (ebays and or Paypals) implosion.
One of the problems with Ebay right now is that it doesn't suit the customers equally. I could go into a whole diatribe about the bias feedback scheme and the other faulty aspects of its sytem, but that is another thread.
The point here is that Ebay NEEDS Paypal more than Google needs Ebay (including its ebay adwords which also leads to its client side adsense).
This was a very smart move for ebay on the short term.
But, it is not a fix. And it is a policy that will likely be overturned.
As Ebay cleans its house (and it is trying), its revenues will increase through its natural auction model. When this happens, it will be able to absorb some of the additional payment fees that will be lost by including the G Checkout (remember that the more that is spent with checkout, the more that the user will get in 'free' adword services. It is not unlikely that ebay would be able to negotiate an even better deal through its volume of adwords purchases and the volume of capital it would fetch over the Checkout).
Overall, i would call the whole strategy very similar to that of the little Dutch boy with his finger in the Dike (like the dam not the other kind). Its a good fix for the short term but, eventually there will be more cracks and the water will come through.
Both of these bullies are on different sides of the playground right now, it will be interesting to see the fallout when they meet in the middle.
oooh ya!?! ever consider Microsoft might be trying to do something like this also, i mean the checkout thing, it would make cents...(another thread)