This first one applies to everyone:

2397711 – Recommended hotfixes for System Center Virtual Machine Manager 2008 R2
http://support.microsoft.com/kb/2397711

Hyper-V Update List for Windows Server 2008
http://technet.microsoft.com/en-us/library/dd430893(WS.10).aspx

Hyper-V Update List for Windows Server 2008 R2
http://technet.microsoft.com/en-us/library/ff394763(WS.10).aspx

Some other fixes that might be relevant, depending on your setup:

975921 – You may be unable to perform certain disk-related operations after an exception when a hardware provider tries to create a snapshot in Windows Server 2008 R2 or in Windows 7
http://support.microsoft.com/kb/975921

975354 – A Hyper-V update rollup package is available for a computer that is running Windows Server 2008 R2
http://support.microsoft.com/kb/975354

975530 – Stop error message on a Windows Server 2008 R2-based computer that has the Hyper-V role installed and that uses one or more Intel CPUs that are code-named Nehalem: "0x00000101 – CLOCK_WATCHDOG_TIMEOUT"
http://support.microsoft.com/kb/975530