Another interesting point I've discovered:
This only works when the nVidia driver is built against the non-SMP version of my kernel. When I try to install it for the SMP version, it not only fails, it makes the non-SMP version stop working until I reinstall under that kernel.
Fascinating. I'm not really sure why I might want to use the SMP version of the kernel. As far as I know, that's for multi-processor machines, which mine clearly isn't. So I'm happy to stick with the non-SMP version. Again, just thought I'd post my findings in case they're helpful to someone else.