Yup, it’s official, Hyper-V is availables will be listed as an "Optional Update" via Windows Update for all Windows Server 2008 SKUs.  

Cecilia Cole (WSUS Program Manager) has all the juicy details:

Today, April 8, the Windows Update team is making the Hyper-V Release Candidate package available as an "Optional Update" via Windows Update for all Windows Server 2008 SKUs. It will also be published as an "Optional Update" to WSUS…