Taylor Brown of the Microsoft Hyper-V Test Team has formally announced the availability of Hyper-V RC1 on Microsoft Download Center in a post on the Windows Virtualization Product Group team blog.
There’s a Hyper-V RC1 package for Windows Server 2008 x64, which must be installed on Hyper-V server’s (physical machines). It includes the Hyper-V Server components for Full and Core installs, the Hyper-V Integration Components for Server 2008 x64 and the Hyper-V Management Components for Full Windows installs.
Microsoft also released the Hyper-V RC1 package for Windows Server 2008 x86. This package includes only the Hyper-V Management Components for Full Windows installs and the Hyper-V Integration Components for Server 2008 x86. It does not contain the Hyper-V Server components (Hyper-V is x64 only).
More here.