Ben Armstrong: The Hyper-V management tools (MMC snap-in and associated UI) are now available for download:
x86: Update for Windows Vista (KB949758)
x64: Update for Windows Vista x64 Edition (KB949758)
These tools allow you to manage and interact with an installation of Hyper-V from your Vista computer in exactly the same manner that you do on a full installation of Windows Server 2008. Note that you need to have SP1 installed on Vista in order for this to work.