I hope this saves some people some headaches (and reboots!
I have been using nvidia graphics drivers with FC2 since 1.0-6106, no problems so far. I also have the habit of compilinig my own kernel, just to get rid of some stuff I don't use and include some other stuff I do use.
When I did this with latest kernel from RH (2.6.8-1.521), I found out my system was freezing hard everytime
(yes, 100%) I left X -- either to shutdown or even when I logged off back into GDM.
After many attempts (see this thread
on Fedora ML), I found out what was the problem: I was disabling high mem support (since I only have 512MB), and this was in turn disabling "Allocate 3rd-level pagetables from highmem (HIGHPTE)". After leaving these options untouched, I was able to do all the tweaking I wanted with my custom kernel (more details on the Fedora ML thread).
So, to nvidia staff: IMHO this should be documented somewhere on the README...