Broken Gnome 3?
This bug has raised its head for a number of users: primarily those using the nVidia binary drivers, in whatever form. Your graphical login page will work, but logging into the desktop will fail - you'll just get a desktop background, and no fall-back mode.
To verify you have this bug, you can drop to a terminal and check your user's ~/.xsession-errors file. It should have an error something like:
Unable to initialize clutter unable to find suitable fbconfig for the GLX context
Also, if you do "rpm -q gnome-shell", it will say:
gnome-shell 3.0.2-3.fc15.x86_64
A revision lower than that (or, hopefully, higher) is a package unaffected.
Fix It
Step 1.
You need to run, as root:
yum downgrade gnome-shell xulrunner firefox
If you don't have firefox installed, then you wouldn't need that package listed in the command.
Step 2.
As the xulrunner update is a security update, you will also need to ensure it doesn't automatically get reapplied the next reboot. The best way of doing this is to edit your /etc/yum.conf to include the following line:
exclude=firefox-5.0-1.fc15 xulrunner-5.0-2.fc15 gnome-shell-3.0.2-3.fc15
You can revert this change at a later date, but it should not prevent more up-to-date packages being installed, so when fixed updates are available they will still be installed.