Windows – What are the differences between Windows 7 RC and the RTM

According to this article, you will need to have an activated copy of XP, Vista, or 7 RC first:

Now here's the bigger catch: While you won't need to downgrade from Windows 7 RC to Vista or XP in order to install Windows 7 final the first, time, you will need to install one of those operating systems and activate it first if you want to do a clean install in the future. Because the Windows 7 upgrade disc will only work with an activated version of Windows. That means you'll need an activated version of Windows XP, Vista, or Windows 7 RC to start with or the disc will be useless.

http://en.wikipedia.org/wiki/Windows_7_editions
Looks like it is just a difference in licensing between Enterprise and Ultimate.

Interesting to note that Vista Ultimate and Enterprise did have some actual differences in functionality (namely, no Windows Media Center or Ultimate Extras in the Enterprise edition): http://en.wikipedia.org/wiki/Windows_Vista_editions

UPDATE: And here is a thread with several MS MVPs confirming that the only difference between Windows 7 Ultimate and Enterprise is the licensing model.