Re: X server 1.12.1 instantly crashes on connection of HDMI monitor on 9600M GT
Explicitly disabling Xinerama doesn't change anything either.
However, the display works fine with nouveau. Thus I'm tempted to say that this indeed is a bug in your driver, or at least the interaction between it and xorg (d'uh).
(Won't use nouveau though since I need vdpau/etc. acceleration and power management. But sweet goodness, KMS makes the console look so much nicer on a FHD display.)
edit: With no relevant source changes, same for X server 1.12.1 and kernel 3.3.2.