Comment 5 for bug 366937

I've been fighting this same problem since upgrading from Intrepid. I looks like X does not load all the dependencies for glx... here's my xorg.conf which works for my radeon 9600 dual monitor setup. (xorg-xserver-ati-video),

This appears to have solved the problem for me. I can't be sure without some more testing though.

Section "Module"
        Load "GLcore"
        Load "glx"
        Load "i2c"
        Load "bitmap"
        Load "ddc"
        Load "extmod"
        Load "int10"
        Load "vbe"
        Load "dri"
        Load "dbe"
        Load "v4l"

EndSection

Section "Monitor"
        Identifier "Configured Monitor"
EndSection

Section "Screen"
        Identifier "Default Screen"
        Monitor "Configured Monitor"
        Device "Configured Video Device"
        SubSection "Display"
                Virtual 2560 1024
        EndSubSection
EndSection

Section "Device"
        Identifier "Configured Video Device"
        Option "AccelMethod" "EXA"
        Option "AddARGBGLXVisuals" "true"
        Option "RenderAccel" "true"
        Option "AllowGLXWithComposite" "true"
EndSection

Section "DRI"
        Mode 0666
EndSection

Section "Extensions"
        Option "Composite" "Enable"
EndSection