Not often discussed, but very interesting… Microsoft Windows Azure offers, among all sorts of other things, the ability to create and join a DC to your corporate domain!  You can imagine the possibilities.

Install a replica domain controller in Windows Azure

You can imagine the benefits… the question becomes, "is Azure the best place for me to keep one of my Active Directory Domain Controllers?".  There are a multitude of local and national EMC-Powered Infrastructure-as-a-Service Providers who can offer not only DC's in the cloud, but all sorts of other customizable options as well.  A quick look at EMC Cloud Service Providers - Americas.