As some of you already know, my husband, Tom Shinder (formerly of ISAserver.org fame) is now working on private cloud architecture at Microsoft. One of his current projects involves creating documentation and guidance to help organizations securely implement their private cloud infrastructures. He recently published this paper, which discusses private cloud security from the architectural perspective and seeks to define the problem domain, key scenarios, and create a structured approach that you will use to create a private cloud security design.
If your organization is considering a private cloud solution, be sure to check it out: