Is Microsoft shipping Office 2013 Professional Plus early to MSDN and TechNet subscribers? Well, according to my account they are! Right now on my dashboard I can register 2 keys and download Office 2013 Professional Plus in both x86 and x64.
As shown in the screenshot from my TechNet account, a new version of Office 2013 Professional Plus was released today, removing the "preview" brand from the entire install.
Installing the Office 2013 RTM requires you to remove the Office 2013 Preview, as you cannot upgrade from one to the other unfortunately.