Sadly, that isn't a dual boot at all, that is just a special ubuntu build that is running on top of the android system and uses vnc to access it.
If the boot loader is unlocked, there has to be a way to actually install an arm version of linux. Another interesting project is linux4tegra.. the nVidia developers are working on a special linux version that is designed for tegra chips (based on ubuntu, but could be promising for other verisons)
I guess the better question is... Has anyone actually booted another version of linux on the xoom?