JUST RELEASED!!! This is an update to enhance the Hyper-V role's support for processors and virtual machines. Hyper-V will now support support up to 24 logical processors and 192 virtual machines!!!
It's on the Microsoft Download Center and can be downloaded from here: Update for Windows Server 2008 x64 Edition (KB956710).
This update will not be available as a general release through Windows Update due to its specialized nature and the limited number of systems needing it.