I have a blogged about this before - but we maintain a list of all updates / hotfixes that have been released for Hyper-V on Windows Server 2008 R2.  We update this list regularly - and have just rolled out the latest updates.  You can review the list here:

http://technet.microsoft.com/en-us/library/ff394763(WS.10).aspx

You should make sure to check this article regularly as we are always updating it.

We also have a similar list that we provide for Hyper-V on Windows Server 2008:

http://technet.microsoft.com/en-us/library/dd430893(WS.10).aspx

Cheers,
Ben