Fedora 12 (and possibly other Linux distributions) ships with the "nouveau" driver enabled by default. Having this driver loaded prevents the NVIDIA driver from loading, producing an error like the following:
NVRM: The NVIDIA probe routine was not called for 1 device(s).
NVRM: This can occur when a driver such as rivafb, nvidiafb or
NVRM: rivatv was loaded and obtained ownership of the NVIDIA
NVRM: Try unloading the rivafb, nvidiafb or rivatv kernel module
NVRM: (and/or reconfigure your kernel without rivafb/nvidiafb
NVRM: support), then try loading the NVIDIA kernel module again.
NVRM: No NVIDIA graphics adapter probed!
Please consult your distribution's documentation on how to disable the nouveau driver before installing the NVIDIA driver.