After upgrading my Ubuntu desktop with sudo apt-get upgrade, the X11 server was unable to start. Indeed, the AMD Catalyst driver was made unusable due to a missing symbol.

If this happens to you, check the file /var/log/kdm.log and if you see some error such as:

/usr/bin/X: symbol lookup error: /usr/lib/xorg/modules/drivers/fglrx_drv.so: undefined symbol: GlxInitVisuals2D
xinit /etc/gdm/failsafeXinit /etc/X11/xorg.conf.failsafe -- /usr/bin/X  -br -once -config /etc/X11/xorg.conf.failsafe -logfil
e /var/log/Xorg.failsafe.log

Then you have to re-install the proprietary AMD Catalyst driver (AMD just released a new driver yesterday).

After re-installation and a reboot, the dual screen configuration was running again. To configure the dual screen, it may be necessary to launch the AMD Catalyst Control Center with:

$ sudo amdcccle

Check out my xorg.conf file in case of problem.