The following whitepapers have been completed in preparation for R2 launch and are posted on the download center.  Additionally, these documents will be highlighted on the Virtual Server R2 Launch page.

Virtual Server Host Clustering Step-by-Step Guide for Virtual Server 2005 R2:
http://go.microsoft.com/fwlink/?LinkId=55644

Using iSCSI with Virtual Server 2005 R2:
http://go.microsoft.com/fwlink/?LinkId=55646

Why would you use Clustering?  Well it's based on MSCS, so anyone with an existing cluster should find it easy to use.  SAN, iSCSI or DAS platforms are all supported (provided you use WHQL systems).  Any guest running on the cluster can be moved, this applies to non Microsoft OS's such as Linux.  And if you need another reason, well it's free!

So you can now easily move virtual machines (guests) from one host to another, for planned maintenance or during unplanned outages.