I mentioned this in another comment, but my understanding (without having worked there) is that they do not use OS X or their own hardware internally.
I think that things can be quite different between orgs though, with some adopting a more enterprise-y appliance setup (NetApp Filers, InfoBlox, etc.) and others building services more like an Internet company (Linux servers and open source based services).
This has myself and a colleague wondering what Apple run in their data centres. Can anybody hazard a guess? Is it Apple hardware with OSX? Is it custom/third-party hardware running *nix? I seem to remember somebody mentioning Azure not too long ago.