well, since Android is running a linux kernel, it's a lot easier to add in a linux-based userspace like Ubuntu than it would be to add Windows or Mac userspaces.
now, nothing stops Apple from doing a similar thing with their idevices, or Microsoft doing something similar with their windows phone offering.
but Android has a much broader manufacturing base to do this sort of thing with.
So no, I don't think there is a demand for "Ubuntu", but I don't think it matters.
RedHat is no longer interested in the desktop (at least on a commercial basis) and there really aren't that many companies competing with Canonical for this sort of thing, so who else is going to put the effort in to this sort of OS market?