If you installed the Nvidia drivers on Fedora using the Nvidia installer, pay attention to any Xorg update as it might break the glx module. I wrote about this some time ago but I found a new, easier way do deal with the issue.
Basically, instead of reinstalling the Nvidia driver to fix the glx module, all you need to do is manually recreate the module symlink. Note that this would only work if libglx was replaced by X server package updates (xorg-x11-server-Xorg), it doesn’t repair other X modules.
Here’s what you need to do to add the right glx module to X (the following worked fine with Fedora 18 x86_64 and Nvidia driver 313.26):
mv libglx.so libglx.so.orig
ln -s libglx.so.313.26 libglx.so
Just replace the Nvidia driver version with whatever version you have installed and the glx extension should load properly again.