Surprisingly, the site you linked in your first post (
http://www.microsoft.com/windows) did not in fact answer how Apple's Boot Camp handles using an upgrade CD to install Windows 7.
I am very familiar with the links and descriptions of how an upgrade from XP to Win 7 works on a regular Windows computer. My question was about whether or not I am going to need to change out CDs during the install process at a point where the optical drive on my MacBook can't eject a CD. That's what happened when I installed Windows XP to start with (using an upgrade CD), and the only way around it involved "doctoring" a CD to make Boot Camp think it was a full install CD. It was doable, but it was a pain, and I prefer not to have to do that again. In my case, I am willing to try out Windows 7 with an upgrade CD, which I can get at a discount, but if the only way to install it involves getting a full install CD, I'm not going to spend the money. The issue is the specific quirk of using an upgrade CD with the optical drive - it's not really a Windows issue per se.
From BlueDevilKurz's reply, it sounds like this probably won't be a problem this time around, as long as XP is already installed, but we won't actually know for sure until Windows 7 is released. It also sounds as if it probably isn't possible to do a
truly clean install, without XP installed already, which is a pain, but better for the optical drive CD switching problem.