@Lonni: I don't seem to spot Nvidia guys (officially, at least) on the xen-devel list as yet..
If you -are- working on this Xen issue, you'll probably wan't to take a look at the xen-devel thread '[Xen-devel] [RFC] Xen Virtual Framebuffer'.
People are quite seriously talking about developing OSS support.
This of cause would mean Nvidia wouldn't have to develop an Xen-aware driver

, but to get all those nice features working in vesafb and OPENGL, well...
OTOH, one is already working on this, and has previous experience with a 2D system. He seems to get some basic features working, based on a closed ATI driver. Interesting... Nvidia, where art thou?