Release Candidate 0 of Hyper-V was released on March 19th.  Details about the release and instructions for installing it are available here: http://blogs.technet.com/jhoward/archive/2008/03/19/Hyper_2D00_V-RC0-release-is-available-for-download.aspx

Hyper-V RC0 Update (x64): http://www.microsoft.com/downloads/details.aspx?FamilyId=DDD94DDA-9D31-4E6D-88A0-1939DE3E9898

To manage Hyper-V RC0 from another 2008 or Vista management box, you'll need to install one of the following patches:

Windows Vista SP1 Management Tool for Hyper-V RC0 (x64): http://www.microsoft.com/downloads/details.aspx?FamilyId=450931F5-EBEC-4C0B-95BD-E3BA19D296B1

Windows Vista SP1 Management Tool for Hyper-V RC0 (x86): http://www.microsoft.com/downloads/details.aspx?FamilyId=BC3D09CC-3752-4934-B84C-905E78BE50A1

Update for Windows Server 2008 (KB949219) (x86): http://www.microsoft.com/downloads/details.aspx?FamilyId=B7464B44-821D-4A7C-9D9C-7D74EC14437C

I currently have a Hyper-V RC0 installation on a test machine running 2008 Enterprise x64 Server Core, being managed by a separate 2008 Enterprise x86 Full Installation workstation.  The new release of Hyper-V definitely brings a marked performance increase in both VM operation and in management.

NOTE: You must install the Hyper-V RC0 x64 update on any Server 2008 Guest OSes in order for the Integration Components to work.  They are built into the RTM 2008 OS, but they are not compatible with RC0 without the full upgrade.