At our TechNet Event in Columbus, OH recently, Andy asked:

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There is no limitation, Andy.  I think some of the concerns people have are related to licensing.  Can I really take a live virtual machine and move it freely between nodes in a cluster?  The answer is, yes.  Absolutely.  Assuming you’re already licensed to run that virtual machine, you can migrate between nodes in a cluster as many times as you want.

CLICK HERE for more details on Hyper-V in Windows Server 2008 R2

And if you’re comparing licensing options between Windows Server 2008 R2 Hyper-V and VMware vSphere 5, make sure you try out this virtualization cost calculator.