Yea, the Hyper-V RC0 patch is available on Windows update!!
Okay, stop now, relax, and think.
What does this mean to me as an admin?
If you have built a nice fresh and clean install of Windows Server 2008 AND have not put any virtual machines on it - go ahead, patch freely and with reckless abandon.
If you are running Hyper-V as the WS08 RTM and have running virtual machines. Stop!
Read this most excellent post from John Howard (all of it).
In a nutshell..if you have snapshots - delete them now (this merges the snapshots into the base VHD), shutdown the VM and wait for the snapshots to be merged in to the base VHD.
This can take a while, soe you have to monitor the AVHD files of the VM - when they are all gone (poof, they disappear when they get mergd in), then you have a VHD you can walk away with.
The snapshots are not 100% compatable between the RTM and RC0.
Document any non-default network settings you have in your VMs (host too)
The netwoking is upgraded in this release AND you will end up creating new NICs within your VMs which means hardware detection kicks in and gives you a new NIC with default settings.
(And if you have an application installed in a VM that binds itself to the MAC address - make sure you know that MAC address so you can set it again)
After the VMs are ready and shutdown you can patch the Host.
After reboot you may need to recreate a VM configuration file, you may need to recreate virtual switches, you may need to attach a VM Network Adapter to a new virtual switch..
As the hypervisor and the virtualization stack is modified the GUIDs assigned to devices changes. In the Windows world we know that if you move a NIC from slot A to slot B we end up with a new device in device manager (a new device GUID).
The same thing is happening under the hood in this case. Only in this case you are left pullling your hair out.
Oh, and if your VMs won't boot after the upgrade you might want to explore this TechNote: