Great news, Hyper-V is keeping its schedule. Here it is.
Here's some : Hyper-V RC0 to RC1 Upgrade Considerations:
*Saved-state files are not supported between RC0 and RC1 releases of Hyper-V. All virtual machine saved states should be discarded before upgrading to RC1, or prior to resuming virtual machines after upgrading to Hyper-V RC1.
*Online snapshots contain virtual machine save-states and thus online snapshots taken with Hyper-V RC0 are not supported after updating to Hyper-V to RC1. Either apply any online snapshots and shut down the VM or discard the virtual machine save state associated with the snapshot before or after the update to Hyper-V RC1.
*System Center Virtual Machine Manager 2008 Beta does not support Hyper-V RC1.
*New Integration Components (ICs) must be installed for your supported guest operating systems. Integration Components are specific to the build of Hyper-V. RC1 Integration Components for all supported Windows Operating Systems are provided using the ‘Action’ -> ‘Insert Integration Services Setup Disk’ action.