QubesOS is Linux+Xen+Added Trickery on top. So calling that a research prototype may not be entirely correct. I have used QubesOS - besides hardware support and usability there are no real inadequacies as far as I could tell from a general purpose desktop OS standpoint - both of which can be fixed by Apple along with may be some more hardware integration.

[EDIT] Relevant Article (via /.) - http://www.networkworld.com/news/2011/110311-xen-simon-crosb...