It takes a seriously robust server network to handle the kind of traffic and load that it takes to run a large and popular cloud service. Both Microsoft and Amazon offer this sort of robust server backend for companies to use. There is a new rumor going around that claims Apple's iCloud service will use the Microsoft and Amazon server networks. Apparently Apple has asked that the two companies not talk about the deal, which is no surprise given they are competitors.
I would have though Apple would be installing its own server network to handle the workload rather than farming it out. At the same time, Apple may be focusing on the iCloud service and meeting user needs than building out its own network. If iCloud proves to be really popular we may see Apple roll out its own network down the road.
The rumor also claims that Microsoft sees Apple choosing to work with it a validation of the Azure platform. Other reports have said that one reason for farming the iCloud network out to competitors was because Apple didn’t want to be a cloud provider. It is also assumed that Apple will have to use some sort of custom software to manage requests and track data since two different companies and two different technologies are in use.