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.

KB# Description Link

KB950050

 Hyper-V RC to RTM update. Mandatory update! Download

KB956710

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.

 Download

KB956589

 Description of the Hyper-V update for issues that may occur when you manage the Hyper-V role on the 64-bit editions of Windows Server 2008 by using SCVMM

Install this update if the host is managed by SCVMM.		 Download

KB956774

 A Background Intelligent Transfer Service (BITS) client cannot handle files that have paths that contain the volume GUID in Windows Server 2008 or in Windows Vista Service Pack 1

Install this update if GUID paths are used.		 Download

KB956697

 Windows Server 2008 Hyper-V VSS writer is not used during a backup job because of corrupted or invalid virtual machine configuration files

Mandatory update when VSS is used for backups.		 Download

KB951308

 Increased functionality and virtual machine control in the Windows Server 2008 Failover Cluster Management console for the Hyper-V role

Mandatory update when virtual machines are clustered.		 Request update

KB953828

 The NLB host does not converge as expected on Windows Server 2008 Hyper-V virtual machines

Mandatory update when virtual machines are clustered using NLB.		 Request update

KB953585

 Error message when you try to start a Hyper-V virtual machine on a Windows Server 2008-based or Windows Vista-based computer that uses the NUMA architecture: "An error occurred while attempting to change the state of virtual machine VMNAME"

Mandatory if host uses NUMA architecture.		 Request update

KB950182

 A computer that is running an x86-based version of Windows Server 2008 or an x86-based version of Windows Vista may use fewer processors than expected if the number of cores on a socket is not a power of 2

Mandatory update if you are running multi proc virtual machines on a physical processor with e.g. 6 logical processors.		 Request update

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.