License key management for virtual machines running in Hyper-V will become automatic in R2. This is definitely a nice and welcome feature but from what I know now it will only be available if your virtualization host is running Windows Datacenter 2012 R2. Plus the only operating system currently listed to support this feature will be virtual machines running Windows Server 2012 R2 editions.
The feature will automatically activate the VMs OS based on the virtualization host being properly activated in addition to the other requirements mentioned. So there are no keys to manage or worry about except on the host and if the VM moves to another environment, it will need to reactivated for that environment. If Microsoft can get this feature in its Windows 2008 Server editions this will have a much bigger impact.