Update after trying all of my Windows 8 Machines:
I'm 1 for 5 with the 8.1 upgrade. 3 of the failed upgrades were Windows 8 Pro Media Center 64bit devices. 1 failure was on Windows 8 RT (Dell XPS 10). All of the failures were attempted over my home ISP (Comcast). Tried the update multiple times on
each machine (cleared store cache, clean boot, etc). All with same error code.
My Surface Pro with Media Center succeeded while I was at Work (though, off topic, appears to have killed my Type Cover, which works on another Surface, but no longer on mine, while my Touch Cover continues to function on my upgraded Surface ....)