Nobody trusts the Debian build tools! Ah well, neither do I, sometimes. kernel-package had an update this morning so I am trying again with the new version.
Ran X with the 'vesa' driver, everything works fine. So it is definitely an 'nvidia' problem.
uname-r reports 2.6.9n and the symlink /lib/modules/2.6.9n/build points to the right directory.
The /usr/src/linux symlink I made in the past (dunno, everyone else's using it) pointed to an old directory (Linus' infamous linux-2.6.9.final), I fixed it before starting this morning's compile.
Will post if I have any more luck today.