I tweaked the patch available at
to apply cleanly with 1.0.8178 (i also applied patch U012206 to this version) and 1.0.8756 and exported the xen kernel symbol as suggested in the post. Both versions compile and the kernel module loads perfectly, however X fails to start with a "signal caught" error. The card was probed successfully and managed to communicate with kernel module (no errors logged to dmesg). Using -logverbose 5 it seems that X fails after the nvidia kernel sets up shared memory and starts handling interrupts (these are the last two lines logged by the driver before it dies). I tried to boot with noapic, acpi=off etc as suggested in the REAME with no success.
I am using GeForce FX 5200, Xorg 7.0.0 and kernel 2.6.16 (+xen patches) on Gentoo, for the record the opensource nv module works perfectly.