Actually, MediaCenter for Windows 8 *IS* completed, and it's already right on the Win8 media you can download from Technet or MSDN. It's just hidden/inaccessible unless you know how. It's actually another "edition" of Windows and has its own system image on the Win8 media, it's just an edition that they only sell keys for as an upgrade, so they hide the ability to directly install it by default.
Following a procedure shown in this article (http://forums.mydigitallife.info/threads/35650-Creating-a-Windows-8-AIO-with-Media-Center-manually
) and elsewhere on the internet, you can take your MSDN/Technet download of Windows 8 Pro and turn it in to an AIO, All-In-One, disc containing all of the different editions (to get the x86/x64 Enterprise editions on the AIO disc, you'll also have to use the Enterprise edition download from MSDN/Technet. The tool you use to do this is the (free from Microsoft) Windows ADK, Assessment and Deployment Kit (formerly AIK, Automated Installation Kit) to disassemble and reassemble the editions/images on the media.
There are actually 8 Windows 8 editions:
Windows 8 (x86 & x64)
Windows 8 Pro (x86 & x64)
Windows 8 Pro with MediaCenter (x86 & x64) (hidden on the installation media)
Winodws 8 Enterprise (x86 & x64)
Plus a sort-of 9th: Windows RT, which is Windows 8 for ARM CPUs (tablets), has limited "legacy" desktop support (i.e., basically Start Screen & Windows Store [formerly "Metro"] apps only)
What you find people saying (on Microsoft sites and on Wikipedia) is not technically correct: MediaCenter is not an "add-on" to Windows (though it can be thought of that way). MediaCenter is not distributed as an application that's installed on Windows. It's an edition of Windows. With Vista, Home Premium and Ultimate editions were MediaCenter editions. With Win7 is was part of all of the editions including and above Home Premium. With Win8, there is only ONE edition that includes MediaCenter, and it's called "Windows 8 Pro with Media Center". Windows 8 Pro does not contain MediaCenter. Nor does Win8 Enterprise. The reason you can't "add" MediaCenter to Enterprise, is because you can't "add" one edition of Windows to another edition. When you upgrade from Win8 Pro to Win8 Pro with Media center, you're moving to a different edition of Windows, one that's a pure superset. And as it happens, Microsoft will only allow moving to the Win8Pro+MC edition from the Win8Pro edition (plus a 2-in-one upgrade path from Win8 to Win8Pro+MC, which they're calling "Pro Pack"). With Win7, all editions were progressively pure superset of each-other. That was not the case with Vista (business dropped MediaCenter), nor is it the case with Win8 when you factor in the MediaCenter edition.
I created one of those AIO discs, and installed Win8 Pro+MC directly from the install media (on a USB thumbdrive) on an empty hard drive, so I can assure you that MediaCenter has been completed, and I'm running it right now. Those bits came directly off the MSDN/TechNet download for Win8 Pro. However, Microsoft has not yet sold any product keys for Media Center, so there is no way to activate that edition of Windows even with an MSDN/TechNet subscription, so I am currently running unactivated. You cannot use a Win8 Pro product key for a Win8 Pro+MC edition, because they're different editions with incompatible product keys. All of the editions of Windows found on those MSDN/Technet downloads have a "default product key" built in to them (it was the same with Win7) that will not activate. So, when you install without a product key, you actually ARE using a product key, it's just that default key. Some of the places online that were describing the leaked media (before they were available on MSDN/Technet) were listing these keys, but they're nothing special, and they don't buy you anything. The reason they were listed is that by default, you can't install without a product key, but entering these default keys bypasses that limitation. However, the above procedure to create an AIO disc puts the Windows installer in a mode where it DOES allow installation without entering a product key, just as with Win7.
Hope that helps! I spent a weekend learning all of that last month.