Thanks Zander, 2.6.23 kernel image, headers, kbuild... and 2.6.22 libc-dev files. No idea how that slipped through.
Looks like I'll have to wait just a bit longer to use the nvidia driver. There's no 'real' 2.6.23 kernels in Debian yet, and 2.6.22 doesn't work with my computer. (Had to use that a backported install disk just to install Linux on this machine)
But at least I know what went wrong, thanks