Several Ubuntu 8.04 fglrx users are reporting a persistent, predictable, repeatable xorg crash when using an x1600/x1400 configured with dual monitors and BigDesktop. To make X crash completely, all you need to do is click on the secondary desktop background. The crash only occurs when compiz-fusion is enabled. Everything is fine when Desktop Effects are disabled.
I also can reproduced this bug. I switched to Metacity and it went away. It is not a single click on the second desktop for me. I have to click between the desktop and a window about 3 times and X will crash every time.