I had the same problem, too. I am on debian/unstable and I think that it is because I had upgraded my gcc between my kernel compilation and the installation of nvidia drivers.
So I recompiled my kernel again (2.6.3), rebooted on it, and then installed the nvidia drivers. Then I didn't have this error anymore ("Unable to determine the NVIDIA kernel module filename")
By the way, the gcc detection script in the install script of the nvidia driver doesn't work, but this is not really a problem.