I have a buddy who is a development manager at Amazon. He said that they have an internal mandate to start using EC2 for many of their customer-oriented sites, and that amazon.com can also move towards it. I believe Bezos calls it "Drink Your Own Champagne" or something like that.
I am not sure about their internal use of the AWS API (e.g. SimpleDB, SQS, SNS).
My experience with SimpleDB and SQS has been very positive. This is not the case for all situations. Reliability and performance were very good, but latency is higher than launching your own infrastructure (as you would expect).
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