xorg.conf is red herring. I removed the file xorg.conf
$ gksu shows the same error (shown once as the password/command dialog comes up, and once more after root/gnome-termial is requested) -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
