"Well, if they announce AD Services running on Linux, you’ll know that their heart is no longer in the Windows data centre."
I guess you could say this is here now. Samba 4 Server does a pretty good 'PDCEm' or Domain controller repl today. So to keep your Data center 'Linux-pristine', you could (with some elbow grease) achieve this via MIIS or other Dirsync tool. Let's face it, most orgs don't really need a lot more than user/perms/pw sync when it comes to the cloud anyhow.
(expecting to be corrected by non-theorists)