There have been some changes to the WMI management interfaces in the release candidate of Hyper-V from the previous beta release. While there have been no changes to the model, a number of the strings used in different places have been "cleaned up" to provide more "human friendly" values. For instance, in this script I check for a caption value of "Microsoft Virtual Computer System". The the release candidate build this is now a much simpler "Virtual Machine".
So if you have a script / program that you created with the beta and you are seeing issues with the release candidate - you should check the values that you are comparing against to ensure that they have not changed.
Hyper-V, Hypervisior, Windows Server 2008, Virtual Server, Virtualization