Microsoft and Amazon haven't commented publicly about the iCloud service, but speculation is that iCloud will be using the Microsoft Azure platform in conjunction with Amazon Web Services. The article at The Register goes so far as to say that "Apple's iCloud is running on Azure and Amazon. Customers' data is being striped between the pair."
According to a recent article in the online publication The Register, Apple's iCloud service could actually benefit Microsoft and Amazon. Although Apple is promoting its own cloud music service, they will be using existing platforms rather than their own infrastructure.