The idea is not new:

Windows 7 Phone 7 & Windows 7
Windows 8 runs is to run on PCs to phones
iOS is built to emulate the experience on OS/X, phone and table experience
are the same

Why not Unity?

The problem is scalability. That is what they are hoping to tackle over the
next while and with Wayland they have hopes. Nobody says they are going to
succeed, but the concept is not bad and if it works it will have a benefit
for users as more people move to big screen TVs, tablets and smartphones.
Right now it does not seem workable because we do not have scalability
working to that degree due to the limitations imposed by X window system
and certain architectures. Consequently things look too big on small
screens and people are complaining. They are working on it and we should
see progress by this time next year. Wayland Project was attempting to
install Wayland on a tablet in November, but I have not seen the results
yet.

The advantage that Ubuntu has is that it can run on different architectures
and takes less in the way of resources than Windows. Apple is locked to
certain hardware. Microsoft is way behind in supporting different
architectures and it takes more horsepower to deliver the same performance.
Android is good, but it does not run on PCs. The field is wide open, so it
is an opportunity. Tablet and smartphone users do not care so much about
the OS, but are more concerned about performance and apps, so there is no
stigma about using Linux that you encounter with PCs.