We have just released an updated version of the Windows Server 2008 R2 Migration Utilities that now adds support for Hyper-V, plus we have updated the Hyper-V Migration Guide.  You can grab the bits from here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=5c1ec14a-e9d7-46ea-9c6e-73d6bc4219b4

And read the guide here:

http://technet.microsoft.com/en-us/library/ee849855(WS.10).aspx

But what is this exactly?

Simply put – this is a set of command line tools (PowerShell to be precise) and associated documentation that allow you to take a Hyper-V installation on a Windows Server 2008 or Windows Server 2008 R2 computer, and move all the Hyper-V bits (virtual machines and all) to a new computer running Windows Server 2008 R2.

Cheers,
Ben