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The report from InfiniteApple indicates that the iCloud service is providing authentication through a windows.net address, and is serving images through an amazonaws.com address. Speculation is that they are using the third-party services as type of CDN, but that iCloud itself is not based off of the services. There is no indication whether their use is temporary, or part of the long term plan.
Apple has been talking a lot recently about its new iCloud service, and is said to be building huge datacenters to accommodate the large volume of new iCloud users. An interesting side note to the story is that, according to a report from InfiniteApple, they are currently using Microsoft Azure as well as Amazon's AWS cloud services.