Hello Fred,

There is another topic discussing this issue. The problem seems to be with the xorg-xserver update. To temporary solve it you can downgrade your xorg-xserver.
#yum downgrade xorg-x11-server-Xorg
After this your login should work again. If you don't want to downgrade, just disable compiz/desktop effects  and you should be able to login again.
There is a bug open for it, you can follow it at https://bugzilla.redhat.com/show_bug.cgi?id=522863


regards.

Rodrigo Renault