I think that Ubuntu Edgy Knot 2 is the first Linux distribution that enables the Composite extension by default for X.org. While this is really exciting, unfortunately it seems to negatively affect DRI and 3D acceleration when using ATI’s closed-source drivers, fglrx.
I don’t know exactly the reasons of this, but enabling Composite extensions disables ATI’s driver support for DRI and 3D acceleration. The X.org logs show DRI has been disabled, but they offer no clue on what’s going on, so I eventually found tout he answer while digging on the Ubuntu forums.
To disable Composite extensions, add the following lines to the end of
Section "Extensions" Option "Composite" "false" EndSection
This will bring back support for DRI, 3D acceleration and, thus, OpenGL support.