“The Release Candidate 1 (RC1) update to the Hyper-V role provides improvements to security, stability, performance, user experience, forward compatibility of configurations, and the programming model. All users of the Hyper-V role are encouraged to apply this upgrade. After you install this item, you may have to restart your computer. Once you have installed this item, it cannot be removed. This update is provided to you and licensed under the Windows Server 2008 License Terms.”
There is also an update to the Vista Management console for RC1 available here.