Dear Brice,
I just did a full upgrade of my T41p, hence now have the package
xserver-xorg-video-ati 1:6.7.197-1.
If I use the xorg.conf which was working in July with both internal
and VGA displays, it does not work anymore and I get a
"Requested Entity already in use!"
error message.
Starting without the external display (i.e. having only the internal
screen in the ServerLayout section) works fine, and I get a copy of
the internal screen on the external.
You will find attached the output of xrandr, my xorg.conf, and the
logs Xorg.0.log.Default (the working one) and Xorg.0.log.DualXinerama
(the problematic one). Each corresponds to one of the ServerLayout.