If you are running Windows Failover Clustering and Hyper-V then this patch might apply to your environment.
With this hotfix we increase functionality and add more control to Hyper-V virtual machines running in a clustered environment. Here are some of the improvements:
This hotfix will help a lot of our customers who are deploying Hyper-V in a Windows Failover cluster environment. Read the full list of improvements: http://support.microsoft.com/?id=951308
At least I could help one customer today :)