I actually was able to do this last night. It wasn't on my mac, but on another pc. It wasn't the exact procedure that everyone is talking about, but I still think it will work.
I had used an older AIO Windows 7 CD that I had since RTM was leaked. I used it to do the install. It installed and then came up saying that it was not genuine, yada yada yada. I then took my upgrade DVD (bought through university) inserted it, chose upgrade. It took about an hour, but when it was all said and done, it allowed me to activate using the upgrade key.
I know that this isn't exactly the same thing as using the upgrade cd to do the original install, but I bet it will work just the same. I'm waiting on my second copy to come in to try it using the upgrade cd for both installs.