There seems to be a bit of conjecture around the net regarding the whole 'hypervisor' / microkernel architecture thing at the moment. And I have to agree with a lot of it. I can see most OS's splitting off into microkernel architectures, running on hypervisors.
My interest was sparked by this video on Channel9... very cool. And now Microsoft going at lengths to interop with XEN makes so much more sense. Who gives a damn what hypervisor is running underneath... you're still going to need a license for every OS you put on top. And with widespread adoption of virtualisation making the hardware, power and cooling costs drop like never before, I'm sure the number of OS 'instances' is set to go through the roof. I can see it happening already where I work, and I'm sure my company is fairly typical in the large enterprise space.
It's also kind of funny to see the squabbling between Novell and Red Hat with regards to XEN support. Novell will only support SLES on XEN, and Red Hat will only initially support RHEL. Cmon babies, that's no way to promote XEN as an enterprise class hypervisor. Even Microsoft officially supports running certain versions of Linux on Virtual Server 2005 R2 (an application), it's hard to swallow that Linux companies won't support competitor distros running on a hypervisor. Hopefully XEN Source will step in and offer some kind of across the board support for any OS running on XEN, like VMware does with it's products.
And I'll bet when the Microsoft hypervisor drops, they'll be providing support for non-Microsoft OSes too!