Microsoft has released a document with details about Virtual Server 2005 R2 host clustering. This is similar to the solution by VM6 for VMWare GSX in that a virtual machine (guest) is configured as a clustered resource. A regular MSCS cluster can be configured to fail over guests to another node in the case of failure.
Full details here: Virtual Server Host Clustering Step-by-Step Guide for Virtual Server 2005 R2



