Hyper-V Release Candidate 1 for Windows Server 2008 was released on the 20th May. This is a full functionality release and provides improvements to security, stability, performance, user experience, forward compatibility of configurations, and the programming model. Further details and downloads available here:

http://www.microsoft.com/virtualization/ws08.mspx

It has also been announced that Microsoft have been using Hyper-V in production for several months to host the MSDN and TechNet websites: 

http://blogs.technet.com/virtualization/archive/2008/05/20/msdn-and-technet-powered-by-hyper-v.aspx