Xinerama is finally working with an external monitor. Below some installation notes are posted.
Based on the old installation setup, xinerama is working on the Dell Inspiron 510m with the following xorg.conf file.
The KDE display manager KDM in Ubuntuy Edgy does not ship with xinerama enabled. To enable xinerama add +xinerama to the call in /etc/kdm/xserverrc
For the Gnome display manager GDM the screen configuration can be choosen at login. For more information see my old installation notes here
If – after changing from KDM to GDM – you are no longer able to login into gdm replace the GDM configuration file */etc/pam.d/gdm with the kdm version. (And don’t forget to make backups)