If you're already running RC0, the upgrade is really simple. Unlike the Beta to RC0 update, virtual network configurations and virtual machine configurations are now compatible and are migrated in-place. The only gotcha to watch out for is that saved states are not compatible, which includes online snapshots. So please make sure online snapshots are deleted and merged, and your virtual machines are shutdown cleanly before applying the update.
If you are still running Hyper-V Beta, take a look at KB949222 for more information. Although that KB was written specifically for Beta to RC0 updates, the same overall information applies for Beta to RC1.
There is one thing in particular worth mentioning about RC1 - after the feedback received by Microsoft that upgrading the Integration Services for Windows Server 2008 virtual machines was inconsistent with all other operating systems, they listened. Starting with RC1, all virtual machines are equal in this regard. Simply run setup.exe from vmguest.iso (Actions/Insert Integration Services Setup Disk in Virtual Machine Connection) regardless of the virtual machine operating system. Note that you still need to apply the main update to the parent partition though!
The links are here:
Windows Server 2008 x64 (Apply this to the parent partition to upgrade to RC1). More info in KB950049.
And if you are using Windows Vista for Remote Management, here's the tools you need: (KB949587) x64 and x86