Ran into this one today. Problem is I used a KMS key instead of a MAK key.
Doh!
At an elevated command prompt, type the following command, and then press Enter:
slmgr -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (your mak key here)
Reference: http://support.microsoft.com/kb/929826
Thanks a bunch, that did the trick!
I installed Server 2012 Std and it had a KMS key already associated with it. This helped change it to a MAK key – thanks for the help