A new System Center Virtual Machine Manager 2008 KB article is published, that discusses an issue where a Read Only Domain Controller (RODC) can only be used as a Host if it was already a Host prior to being promoted. If a RODC needs to be used as a Host and was not previously managed by SCVMM, it’ll have to be demoted to a member server and made a managed Host prior to promotion back to RODC. Important: Although a Read Only Domain Controller can be used as Host in SCVMM, this is not the recommended usage of a Read Only Domain Controller.

More info: KB975113 Using a Read Only Domain Controller as a Host in SCVMM 2008