Website now says up to 72 hours for key arrival. I got my first key, a couple of weeks ago. My second key (different e-mail address) is currently taking 6 days and no sign of it so far. Reapplied 3 days ago for second key.
I got my second copy of Windows 8 from Newegg. Considering the article I read about Media Center Keys validating certain types of installs, I wonder if there is a problem getting keys for Windows 8 installs not purchased and verifiable through Microsoft.
Oh, and using the same WMC key for a second install does not seem to be a good idea. Someone said you could call Microsoft, but it will deactivate the current Windows 8 install when used on a second computer.