Hello,
Are you using Hyper-V RTM or evaluating Hyper-V R2? In Hyper-V RTM you need to allocate one LUN per VM to allow for the clustered migration process to function properly. The LUNs are moved from host to host, allowing for quick migration and failover processes.
In Hyper-V R2 you can use Clustered Shared Volumes. Here is an article overviewing this technology, which allows for Live Migration:
http://technet.microsoft.com/en-us/library/dd446679(WS.10).aspxNathan Lasnoski
http://nathanlasnoski.spaces.live.com