John Savill (formerly of NTFAQ.com) and currently of Windows IT Pro's "FAQ for Windows" just added a new topic on defragmenting Clustered Shared Volumes (CSV). As if IT doesn't have enough 3-letter acronyms already, CSV is a major feature in the upcoming 2008 Server R2 releases (aka Windows 7 Server).
CSV allows write access to a given VHD from more than just the one host node, and quite a few other enterprise-virtualization necessities. CSV is the backbone technology that allows for the new "Live Migration" feature. R2 is a major step forward to making Microsoft's virtualization solution more "enterprise-worthy", and a major step forward to supporting VDI on Hyper-V.
Here is a link on setting up and using CSV.