Metro UI probably is not a fullscreen application running on top of the standard Explorer.exe UI indeed. It's probably on the same level as explorer.exe if anything. They've probably finally come up with a way to load/unload UIs dynamically (while it was always possible with Windows NT, it was never configurable as such nor was it ever supported. Sure you could always just kill explorer.exe, bring up task manager and then run a new UI process like litestep or progman or whatever...).
If only i got a penny for each time i had to kill explorer.exe.... but yeah, thats what i assume they did. Not that they've been that clear on the Metro, but we have statements about the desktop "just being another app".
But as far as OS design goes and comments I've seen here, no, it doesn't run in the "Core". It's a pluggable module on top. We're not in 1985 here, OSes are modular and very much a whole made up of parts. Here's the Windows NT 4.0 architecture diagram describing API sub-systems for instance (first one to come up on a quick google search, not bothering to find an updated version for Windows 7) :
Wont argue against that. If it was anything but modularized i would be very surprised.
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I'm sure we could find one for the graphics display sub-systems from kernel to user interface if we digged far enough (not interested in doing it, just run Linux if you want to know about these things, it's transparent about such things in way proprietary OSes are not).
My interest in architecture related issues is quite slim, as i am in the research side of ICT rather the more hardcore dark side of things :- )