Hello Kappy,
I'm glad I found your post on this as it seems to almost cover my scenario... Almost :(
I own a MacBook Air and my wife owns a MacBook Pro. My main computer is, therefore, the MBA. If you checked my App Store activity, you could see that I buy stuff 90% of the time from the MBA. However, I sporadically use my wife's MBPS on which everyone of us has a different user account.
When ML came out, I upgraded first my MBA, and then I went on to upgrading hers. In both cases I used my own Apple ID since I'm the administrator on both devices. My user has full access rights on the two computers, she is a plain user.
Now we've just sold the MBP. Yesterday I wiped it clean but I got stuck in the reinstallation process when I was prompted for my Apple ID.
How does this license work in my particular case? I don't want to lose the right to run ML on my main computer but at the same time I have to get rid of the MBP with a OS (whichever it is). I can't seem to go back to Lion anymore.
Thanks in advance for your time.
Marc