There is a new Hyper-V Security Guide available for download now.  This is a great read that goes over the practical steps that Hyper-V administrators can take in order to ensure that their servers are securely configured.

You can download it here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=2220624b-a562-4e79-aa69-a7b3dffdd090

Cheers,
Ben