Glad to see this is finally happening.
The vision of having a completely uncompromized mobile environment transform into an equally uncompromised desktop environment when docked with a keyboard and or mouse is very interesting. An operating system that adjusts itself based on screen size and the input devices attached to it.
Plug your phone into a dock connected to a monitor, mouse, and keyboard and it becomes a desktop. Plug your tablet into a keyboard dock and it becomes a desktop. Disconnect and it's instantly back to tablet mode. No fussing with different screens, silly start buttons, or a touch interface that also has to be mouse compatible.
Also, since most of the Ubuntu app repository is already optimized for ARM, there are very few compatibility issues that way.
I'm not sure how well received another mobile os will be, but Ubuntu Touch has generally received some pretty good reviews from testers.