D'oh! I forgot about that one. Edit the top-level Makefile in /usr/src/linux, and change EXTRAVERSION=-14custom to EXTRAVERSION=-14 (i.e. delete the custom part). You may need to make mrproper oldconfig dep again, but I don't know for sure.
Do another make clean and make on the nVidia drivers, and see if that helps.
The problem here is that RH assumes that the only thing anyone would want the kernel sources for is to recompile the kernel, so they changed EXTRAVERSION to avoid overwriting their /lib/modules directory when the modules get installed. But if you only need the kernel source installed to be able to compile external modules, you need EXTRAVERSION to be the same as your running kernel's EXTRAVERSION, which is -14.