Jeff Wolsey talks on the Edge about what's coming in the RC0 release of Hyper-V, described in this KB, including:
As of today, you can get the RC 0 off WSUS, since it is a Windows Server 2008 patch. For more info see How to Install Windows Server 2008 Hyper-V Release Candidate.
You'll want to read the Release Notes for This Release of Hyper-V.
You can find the Hyper-V manager mmc here:
Vista x64 Edition: http://www.microsoft.com/downloads/details.aspx?FamilyId=450931F5-EBEC-4C0B-95BD-E3BA19D296B1&displaylang=en
Vista x86 Edition: http://www.microsoft.com/downloads/details.aspx?FamilyId=BC3D09CC-3752-4934-B84C-905E78BE50A1&displaylang=en
NOTE: Virtual machines that were created on the beta version of the Hyper-V role do not start after the Hyper-V role is updated to a later version. You can use the virtual hard disk (.vhd) files. However, the virtual machine configuration settings must be re-created. Stepwise instructions are in KB 949222 (http://support.microsoft.com/kb/949222/)