Since the release of Hyper-V there have been several important updates. (Updates till September 2008)
The following table provides an overview of when to use which update.
Hyper-V update : A Hyper-V update is available to increase the number of logical processors and virtual machines on a Windows Server 2008 x64-based computer Install this update if the host has more than 16 logical processors.
More info on KB articles related to Hyper-V on (in the community section): http://technet.microsoft.com/en-us/library/cc754747.aspx
Thanks to our colleague Joachim Gomard for providing a lot of the content.