Been There - Done That!
I too did my Windows 8 and Media Center upgrades the first day. It took about 36 hours to get my first Media Center license due to the heavy access initially. The trick was you can only get one license string per email address. It was necessary to request the others using my gmail and yahoo addresses to get the licenses for my other two computers. All in all, everything went well considering the new launch.
It will be necessary for you to retain both your Win8 key and your MC key if reloads are needed. If you're having any problems, call Microsoft. They were very helpful even during the beginning of the Windows 8 launch.
My recommendation is to Custom install Windows 8 into a new partition, keeping your old partition available to browse for earlier drivers if needed for some older hardware. My Windows 8 boot times after removing the dual boot configurations is less than 15 seconds on all three of my computers.
