Decided to replace my FC5 install with FC6 (what an idiot) and expected trouble with the nvidia driver as usual, but this was a bit beyond the norm.
Here is a summary of what it took for me to get it to work in case it helps someone else; this is derived from lots of hints from other messages around the place, but I could not find a consolidated set of instructions.
Bottom line is xen kernels will not work, linux/config.h is no longer present in kernel development sources and nvidia installer expects it, and explicit specification of path to kernel sources is required since nvidia installer cannot find them.
Installed fresh FC6 from DVD image with ALL packages selected
# yum -y remove kdegraphics-devel kdepim-devel # else yum update fails dependencies
# yum -y update
# yum -y install kernel-devel
# yum -y install kernel.x86_64 # since only xen kernel was installed
# vi /etc/grub.conf
... edit to use the non-xen kernel just installed as the new default
# wget http://download.nvidia.com/XFree86/L...-8776-pkg2.run
# sh NVIDIA-Linux-x86_64-1.0-8776-pkg2.run -x
# cd NVIDIA-Linux-x86_64-1.0-8776-pkg2.run
# vi ./usr/src/nv/nv-linux.h
... edit to remove #include linux/config.h
# init 3
# ./nvidia-installer --kernel-source-path /usr/src/kernels/2.6.18-1.2798.fc6-x86_64
... successful compiles, loads modules, but crashes trying to make xorg.conf
Just used the old FC5/nvidia xorg.conf that had been working before and it still works just fine.
Tried the beta 9625 driver and it works too ... this time the xorg.conf phase did not crash (whether having in place the xorg.conf from the initial FC6 install, or my old one, before running the installer).