It turns out that Microsoft's MSDN Direct Student Download program is one of the customers using the Amazon S3 service. From a press release:
"We needed a storage and delivery solution that made it simple, fast, and dependable for students in hundreds of countries around the world to download our software at any time," said Joe Wilson, Director of Academic Initiatives in the Developer Marketing division at Microsoft Corp. Microsoft wanted to scale the program up without any upfront or increased ongoing expenses, which is why it chose Amazon S3. Microsoft expanded the program while managing to cut storage costs by more than 90 percent since switching to Amazon S3. "In addition to being easy for our users, Amazon S3 allows us to deploy and scale up in a very cost-efficient manner," said Wilson."