I awoke this morning while on the road in Sydney Australia to the news that Amazon was switching search providers from Google to MSN. I agree with Danny Sullivan’s analysis here on Search Engine Watch, that while the A9 switch is interesting, the greater impact will probably come from Amazon’s serving of MSN listings from within their main domain versus the previous arrangement with Google.
Hitwise clickstream data can provide us with some insight into the magnitude of this decision, both from the perspective of Amazon’s downstream to Google as well as Google’s upstream to Amazon.

At over 8%, Google is the #1 downstream site for Amazon. This is most likely the result of Google sponsored listings appearing within the Amazon domain as well as some non-referred traffic from Amazon to Google. It’s also interesting to note that the downstream percentage to A9 from Amazon is only 1.8%.
The below clickstream report illustrates where Amazon ranks in the upstream for Google. While only 0.5% of Google’s traffic it still a significant provider in the #8 position overall.