I made a patch file for this driver from Luiz Vitor Martinez Cardoso [Grabber]'s post
to the xensource list, he gets all the credit.
Apply via running 'patch -p0 < patch-xen3.0-nvidia100.14.11.txt' while in the usr/src/ directory.
Verified working on Ubuntu Feisty amd64 w/ latest updates.
Let us know how it works for you.