Hi, 
 
I recently upgraded my X11 from XFree86 4.3 to Xorg 6.8.2 (Debian unstable). 
Now my xinerama setup doesn't work anymore. I have a laptop with an M9 and use 
the laptop display together with a crt monitor in xinerama mode with a display 
size of 3360x1050 pixel. Now of course the crt doesn't support 1680x1050 pixel, 
thus I used the maximum resolution that it can do, which is 1600x1200 pixel and 
setup a virtual display on that screen. This gave the smalles possible 
"scrolling" and was working perfect until I changed to xorg. Now the 
radeon-driver tells me that 1600x1200 is not a correct mode and sets the crt to 
1680x1050 using 60Hz vertical refresh. For now my workaround is to use 
1280x960, but this no end solution, because xorg should allow me to use the 
maximum resolution the monitor supports (and he does support 1600x1200 at 
75Hz). 
 
I attach the Logfile and the Config. 
 
Andreas