Of course, this still doesn't explain why xinerama suddenly stopped working. Could you test booting the gutsy live-cd, apply your xorg.conf, restart X, and see if you can recreate the issue in that environment? If so, then also test against the tribe-5 live-cd. This will help rule out if recent (post-tribe-5) work is what caused the issue.
Michael,
-nvidia also has a TwinView mode (see http:// ubuntuforums. org/showthread. php?t=221174 for directions) you could test.
Of course, this still doesn't explain why xinerama suddenly stopped working. Could you test booting the gutsy live-cd, apply your xorg.conf, restart X, and see if you can recreate the issue in that environment? If so, then also test against the tribe-5 live-cd. This will help rule out if recent (post-tribe-5) work is what caused the issue.