Hyper-V RC0 is out
Details and download bits can be found here.
Just some additional details. If you have existing Windows 2008 virtual machines, you need to download this and package it into an ISO to mount it into the virtual machine's CD-Rom drive, as you will lose your network connections after upgrade.
For your existing windows 2003 or windows XP machines, you just need to update the Integration Services
Updates: For windows 2008 x64 virtual machine, you need to install the same bit you installed on the host before you get your network back.