I'm currently running the fglrx packages on debian unstable on 2
monitors using "Big Desktop" mode, and have noticed 2 things that I
can't explain by the lack of Xinerama extension implementation.
1) xscreensaver hacks only display on the left (first) monitor, and
the right monitor displays the desktop, thus negating any screensaving
effects on that monitor.
2) open GL apps seem to draw about 1/2 screen to the left of where
they think they are supposed to be. This means openGL xscreensaver
hacks take up only the left half of the first monitor, and glxgears,
etc don't display in the window unless I resize it to be very large,
at which point it still doesn't display in the correct location, but
does display.