NVIDIA kernel module can't be built with dkms - says kernel source is not installed

[Claude Jones]

I'm trying the Freshrpms NVIDIA driver package - it installs 
without incident, but, when I reboot, it gets to dkms and says it 
can't build the module because kernel source is not installed, 
but:
$ rpm -qa | grep kernel
kernel-headers-2.6.18-1.2798.fc6
kernel-devel-2.6.18-1.2798.fc6
kernel-2.6.18-1.2798.fc6

I seem to have seen some references to borked kernel headers in 
the past days on this list, but I can't seem to find specifics, 
or a fix. Is this what I'm running up against?
-- 
Claude Jones
Brunswick, MD, USA