Update: James Stephan, currently Senior Analyst with Dell Health Services, was kind enough to point out to me that I had neglected to mention this procedure will only function with fully licensed versions of Windows Server 2012. If you have downloaded and installed the free edition of Windows 2012 Server Core, you cannot activate the GUI. For quite a bit of detailed information specific to the free edition of 2012 Server Core, follow this link to James' blog.
So I just started playing with Server 2012. Right out of the gate, I encountered issues on installing to a hard drive with a pre-existing Windows 7 installation. I nuked the partitions during the install, however when trying to install the full server GUI, I got a "Windows component cannot be found" error. I believe this was the result of the installer attempting to salvage data from the existing partitions, I still don't know (if you do know, please shoot me an email).
The installation media image was intact, so instead of banging my head on it I reinstalled server core only, and activated the GUI component following the installation.
If you have installed the full GUI from the installation media, you can activate/deactivate/reactive it by launching sconfig from powershell or the command line as Administrator and selecting option 12. If you have not installed the full GUI first, there is no option 12 displayed in sconfig.
Easy fix. Launch powershell from the command line, and use dism to activate the component (this assumes your installation media is on D:\ but you can replace with any path to your installation ISO):
dism /online /enable-feature /featurename:ServerCore-FullServer /source:d:\sources\sxs
This will take some time to process, so be sure to be patient (it took about 10-15 minutes for me). Do not reboot following completing the command. First, activate the shell:
Now reboot, and you're done.