There is an update available that allows your KMS server to activate Windows 8 and Windows Server 2012 operating systems.
An update is available for Windows 7 and Windows Server 2008 R2 KMS hosts to support Windows 8 and Windows Server 2012 http://support.microsoft.com/kb/2691586/en-us
I have attempted this upgrade and it doesn't work. I've confirmed with KMSADD@microsoft.com that the key I'm using is valid. I installed the hotfix and uninstalled the previous Host_B key, then installed the new key. I get "Error: 0xC004F050 The Software Licensing Service reported that the product key is invalid" I can reinstall the old version key with no problem. This is getting very frustrating and I'm begining to feel that the KMS system is more trouble than it's worth!
@Warren - Review this information <technet.microsoft.com/.../ee939271.aspx to make sure you are using the correct key. If you are still stuck you should open a support case at 1-800-936-3100. Trust me, if you have a large environment with thousands of computers and servers KMS is worth the effort.