I take issue with the statement that DisplayLink's USB2 adapters do not work easily on Linux. In ubuntu 12.10 with the kernel updated to 3.6+ (and I assume in ubuntu 13.04 stock) running KDE, if I plug in a displaylink adapter a dialog pops up asking me if I would like ot automatically use the new display plugged in. By default it operates in clone mode, but it is quite simple to use the display preferences panel to use it to extend the desktop.
Currently to my knowledge Ubuntu is the only distro to support this, because they are using an early version of xrandr 1.4 announced here:
http://lists.x.org/archives/xorg-ann...ry/002170.html - now that this is released, any distro with current versions of the kernel, xorg, and supporting libraries will have support for using screens from multiple GPUs in the same display.
This honestly works incredibly well - I have been using it since last august, though at the time it required building and installing my own versions of unreleased software. Right now my desk has 4 displays driven off of one laptop - 2 displaylink, one vga, one dvi.