I ran a yum update on my Fedora13 install the other day, and then when I booted back up and logged in, my KDE would not start! It would start to show the login screen, but once I did a login, it blacked out and went back to the login screen again.
Upon checking my .xsession-error file, I saw:
ksmserver: smybol lookup error: /usr/lib64/libGL.so.1: undefined symbol: _nv000027gl
It took a little looking around, but basically to fix this, you need to point to a proper Nvidia driver file. I ended up removing the old link and making a new one. My machine is x64, hence the lib64 directory instead of just lib:
root{/usr/lib64}: rm libGL.so.1
root{/usr/lib64}: ln -s nvidia/libGL.so.1 libGL.so.1
That did the trick!