You want your devices to work consistently, but take advantage of the special hardware you're using. For that reason, I think one interface for tablets and laptops (as an example) is suboptimal, because you end up compromising on both sides. Some of the complaints about gnome-shell and Unity are I think symptomatic for these compromises.
In Plasma Active, we're sharing components and infrastructure across devices, but create specialized interfaces suitable for the type of device (screen, input methods, etc.), and the use-cases that go along with it to get the best user experience out of your hardware. The end result will (hopefully :-)) be a system that works consistently across devices, integrates well with your other form-factors and is easily adaptable to new types of devices.
And yes, the plan is to also support smartphones at some point, but given the "closedness" of this segment of the device spectrum, we are concentrating on tablets first -- a lot of that work also benefits an eventual phone interface at some point in the future, and not to forget the desktop and netbook interfaces.