Unit Testing support should be included with all versions of Visual Studio 2005 and not just with Team System.
If you want to read more about this, go here.
The Unit Testing stuff in the new Visual Studio looks awesome, and is something that could easily be a foundation piece for all versions. In other words, the architect and team versions do not depend on this feature to define them.
I am not a big NUnit, or Unit Testing guy. However, when I have used it in my Python code, it has saved me time and time again. I think it is rapidly becoming one of those essential things that all programmer IDE packages will have.