This is pretty cool -- I didn't even notice this at first! Hyper-V RC1 is now available for download through the Microsoft Download center or through Windows Update as an optional update. One of the nice changes here is that you now install the Hyper-V Integration Services on Windows 2008 guest machines the same way as any other operating system (before, you'd have to install the Hyper-V patch itself as a separate action).
That would be why my Windows Server 2008 machine wanted an extra reboot this afternoon...