I've been mucking around with Vista (formally Longhorn) for about 3-4 years. This means that things like "all editions are on the one DVD" is pretty much ingrained in my skill, however sometimes I forget that this was not the norm previously and sometimes trips people up. 

I got a mail the other day that said (edited a bit and published with permission of the sender):

I installed Vista Business on my PC. I read in your blog that if you use a genuine Vista DVD (including a MAPS version), then you would not need the upgrade disk. I tried to upgrade to Ultimate. I made the purchase online and and attempted the upgrade. I need the disk. I went back online and ordered it and now I am waiting to upgradewindowsanydaynow. Did I do something wrong?

The answer was very, very simple - take the downloaded utility, run it and when it asks for the Vista DVD, put in the same one you already have.  However, since I have to admit that I have NEVER actually bought an upgrade key online I replied:

You absolutely should not require the disk, can you describe the process to me - it should be something like:

1. You may the purchase
2. Download the code
3. Run the code
4. Insert your Vista DVD
5. Let the install happen.

Could you a) test the process and b) send me a screenshot of the error(s).

Being the lucky (or well researched) person, I quickly got the reply back:

Despite all my experience and training, I felt uncomfortable when it came to launching the original Install again. It just didn't seem intuitive. I felt it should have been more evident as I was inserting the Vista disk that it would lead to an upgrade.

Got Ultimate installed now.

THANKS!!!

It does work - it may seem a bit odd that on the one DVD, contained in the 1 WIM file are all the files required for Home Basic, Home Premium, Business, Ultimate, all the "N" versions and recovery information.  However that is what a single instance manager can do for your disk copying technology (i.e. the ImageX technology used by Vista).

ttfn

David