I also used to be able to spread a Blender window across two monitors without issue. After one of the previous driver releases it broke that and "double screening" the window would crash the machine.
With some superficial testing it looks like I can do it again, except that the window decoration is corrupted when the window is across two monitors. I wonder if it's Compiz or FGLRX at fault with that one.
Just resized a Text Editor window across screen boarders which dumped my X session back to GDM.