Since Office 2013 has gone RTM, I decided to start playing around with it a little more. I downloaded the ISO from MSDN and, being a system admin, immediately got started on creating an automated install. I extracted the Office 2013 Pro Plus ISO and ran setup.exe /admin. I immediately got the message 'Files necessary to run the Office Customization Tool were not found. Run Setup from the installation point of a qualifying product.'
I was a little surprised, but a quick search turned up an artilce for the same issue with Office 2007 that occurs when the admin folder is missing from the installation point. I downloaded the Office 2013 Preview Administrative Template Files from http://www.microsoft.com/en-us/download/details.aspx?id=30341, extratced admintemplates_32.exe since I am installing the 32 bit version of Office and copied the admin folder to the installation point.
Now running setup.exe /admin produces the expected wizard for the Office Customization Tool.