One of the new features of Windows Server 2008 R2 with Hyper-V role enabled is that the virtual machines running on the host can be protected against a host failure by clustering the host. However, when using the core edition you might run in a catch22 situation when all hosts in the cluster were shutdown (for maintenance, power failure) and you want to reboot your servers. If your active directory domain controller and DNS are running on virtual servers hosted on the shutdown cluster nodes, the servers will not be able to boot. The cluster needs an AD and DNS server.
Read all about it at http://blogs.technet.com/askcore/archive/2010/01/14/be-sure-to-plan-carefully-when-virtualizing-your-infrastructure.aspx