I heard it's written in QuickBasic. I've downloaded the source in Longhorn.bas. I will make it available if you want it.
Better question is, is the .NET somehow more closely tied to Longhorn than what it is in XP? Hard to say, but you can be sure that the framework is always in the memory - most likely even when the user has not even logged in yet.
Isn't the WinFX layer written in managed code?