Just an update. I've installed fedora 18 twice now. I go to system-settings -> Display.
This is the odd part, I can see the second monitor, however, when I turn it on the machine freezes (?kernal panic?).
I am forced to hard-reboot the computer. Now, the second monitor is not present in Display... (I've tried all permutations of shuting down computer whilst monitor is on/off etc.).
I reinstalled fedora 18 to the same machine and the exact same freeze occurred when trying to turn on the external monitor.
The second monitor is no longer visible in system-settings -> Display.
(Does anyone know how to see system messages that may have captured logs on the freeze?)