Besides breaking the Intel's stuff, we have also been doing other things over at our lab. Thought I would share this cool screenshot of a Virtual PC running inside Xen. More precisely what you see on the pic is: Windows XP running inside Virtual PC, that runs inside Vista, which itself runs inside a Xen's HVM VM. Both the Virtual PC and Xen are using the Intel's hardware virtualization (VT-x is always used for HVM guests on Xen).
Our Nested Xen patch is part of a work done for a customer, so it is not going to be published. Besides it is currently a bit unstable ;) It's just a prototype that shows such a thing could be done.