DisplayLink dual-head on Ubuntu
After a few iterations, I finally got dual-head working with Ubuntu (Lucid) on a Mac Mini (booted by rEFIt), using an IOGear GUC2020DW6 USB/DVI "DisplayLink" adapter for the second head (1280x1024 17" monitor). The main head is a 23" wide-screen at 1920x1080 connected to the built-in Intel adapter.
It was very much a trial-and-error process, so I don't really have a clean set of steps to follow.
Although I tried building some components from source, I believe that Lucid actually has all of the necessary bits available. I did have to install the optional package xserver-xorg-video-fbdev (Xorg driver).
The key to getting this to work was to make the DisplayLink device the primary screen (0), and the built-in Intel adapter be secondary to that. I'd really rather it was the other way around, but with that arrangement, the DisplayLink device never activated when the X server started.
Other possible contributing factors include enabling XRandR and disabling Xinerama (both via xorg.conf).
I created xorg.conf by using a virtual control (Control-Alt-F1), stopping gdm (service gdm stop), and running Xorg -configure. Copy/move the resulting file to /etc/X11/xorg.conf, then add the bits for the DisplayLink device, etc. My resulting file is attached.
I had confirmed that the DisplayLink was /dev/fb1 as follows:
mini-ubuntu ~ % cat /proc/fb
mini-ubuntu ~ %