>Lets point out the obvious: the user interaction of a small handheld device is very different than that of a computer with a full size monitor and mouse.
And what happens in-between? What happens in the case of tablets? Notebooks with a touchscreen? Future devices like large format displays with full UI, and future paradigms like gesture control?
At this stage the key differentiation point is touch (ie. your finger as the pointer) v. the pointer (mice/keyboard, etc.) The UI for these two categories is naturally different, and convergence would be possible only when the UI can handle both. Even in the case of notebooks with touchscreens, the UI has to handle both requirements well, that of the finger and of the mouse/keypad. I personally feel Ubuntu is neglecting the latter. In time, that may well be the right choice. At present, it's definitely only part of the story.
I think the answer re tables is the meta-answer for linux generally: copy somebody else who has solved the problem. The Apple IOS-OSX dichotomy comes to mind as something to ape.
You just make another couple of custom UIs for "in-between" cases.
Notebooks with a touchscreen can be treated either as tablets with a bunch of integrated USB/Bluetooth peripherals, if you want to use them like MS Surface, or alternatively, a regular notebook with an oversized multitouch-enabled trackpad that happens to be the display. So there's no need for a new UI paradigm there.
Super-large or gesture-controlled devices will probably need their own UI paradigms when they become mainstream.
None of this is meant to imply that every device will need its own, incompatible, radically different UI. Competent designers should be able to come up with a "line-up" of 3-4 UIs that "gracefully degrade" into one another, e.g. a menu bar appears if the device has a mouse, and non-maximized apps are allowed if the screen is larger than X inches. And of course, lots of options for power users, e.g. I want non-maxed apps in my 7-inch device even though the default only triggers them in 10-inch devices, and I want Alt+Tab to switch between windows rather than apps.
And what happens in-between? What happens in the case of tablets? Notebooks with a touchscreen? Future devices like large format displays with full UI, and future paradigms like gesture control?