I managed to get TwinView working on the Dell Latitude D620 under Debian GNU/Linux and I’ve been using it for a few weeks now without any problem whatsoever.
The key section in /etc/X11/xorg.conf is this:
Identifier “Generic Video Card”
Option “MetaModes” “1280×1024, 1440×900; NULL, 1440×900″
Identifier “Generic Monitor”
Identifier “External Monitor”
I’m using an external 1280×1024 LCD, and I use the KDE control center (Peripherals/Display) to switch between metamodes. The nice thing is that whenever the X server starts and it doesn’t sense the external display it automagically defaults to using only the built-in display. Moreover, TwinView handles gracefully the “hole” below the 1440×900 display. Now what I’d really like to do is to specify that the primary monitor is the built-in display instead of the external LCD, but that can wait a bit longer.
Of course, you can download the complete xorg.conf I’m using, with my finely-tuned (right) TrackPoint and TouchPad configuration.