Look at these photo.
Those look to be HP Proliant servers.
Apple is a consumer product company. Any corporate inroads have been ancillary to selling consumer products. It's server products have also been geared to small scale operations. Apple historically has used some of its own products for server operations. For example, XServe and Web Objects as the front end for iTunes. However, it also uses other companies server and data base products. Once Apple started buying data centers, it isn't hard to understand why it dropped X-Serve. It wants to focus on the front end for consumers, not the back end.
Some sites suggest Apple used Solaris for MobileMe.On its jobs site,
Apple claims its data center environment consists of MacOS X, IBM/AIX, Linux and SUN/Solaris systems.
Further, it states one center will have a heavy emphasis on high availability technologies, including IBMs HACMP and HAGEO solutions for high-availability clusters, Veritas Cluster Server, and Oracles DataGuard and Real Application Clusters.
Job prospects are also asked to know storage systems using IBM, NetApp and Data Domain, and data warehousing systems from Teradata. Networking positions require a familiarity with Brocade and Qlogic switches.
As Apple focuses on consumer products, it might as well rely on companies for the back end that have a proven track record in server solutions, which has never been Apple's strengths. I doubt Apple is using Microsoft's or Amazon's server farms unless the demand is larger then Apple's three farms can deliver. For instance, it probably would just be licensing the product to use on its own servers. The choice of Amazon wouldn't' be hard to understand either, since Amazon was one of the pioneers of cloud computing.