This is Really Old News. Been around since (IIRC) NT 3.1.
Same kernel, different tweaks. Basically, most of the software is there, and the NT kernel checks the registry on boot to see which way to set itself up. A lot of the "performance enhancments" that they're talking about with .NET server is the fact that it starts with the shell UI cruft OFF, instead of ON by default. :)
Oh, and XP doesn't have the RELEASE version of .NET server - that would kind of defeat the point, no? :) I'm not surprised that .NET Server has a complete build of XP in it, though. Microsoft has never really tried to change their code behaviors to match other people's wants - just hidden it a little better.
I agree with the whole "licensing" bullshit - and I'd feel more hostile towards M$, if everybody else didn't do it either. (Except for Linux)