We do all of our design and development in the open. We have the plasma-mobile repository that holds things specific to the Active shell. The rest of our code can be found in the kdelibs, kde-runtime, kde-workspace and kdeplasma-addons repositories.
Design is done collaboratively on the active at kde.org mailing list, #active on IRC and occassional VOIP/video calls. The core team gets together in person every few months as well to sync up, and we hope to grow those over time.
We're very open to people taking our work and making something different with it as well. git and our general open door policy makes this very easy. In fact, we hope that over time people making various sorts of devices, from tablets to set top boxes to phones to netbooks, will do just that. Differentiation with compatibility due to a common framework.
We're also very open to 3rd party applications and are hungry to see to more touch-friendly F/OSS applications join us in building up a truly open device ecosystem. Your applications can help define what tomorrows devices are capable of.
QtQuick / QML
While the bulk of our interface work in Plasma Activfe is being done with QtQuick technologies such as QML, this is not a requirement for 3rd party stand-alone applications. We recommend QML, but it's not a requirement. We feel it is unrealistic to expect many applications to suddenly jump into the QML world today. Many can be made touch-friendly and device ready with fairly minimal changes to the existing code base. Still .. QML is pretty impressive stuff, and you may want to check it out! :)
Plasma Desktop / Netbook
What we learn about using QML in Plasma Active will eventually impact future releases of Plasma Desktop. We're already using components in both directions (from Active to Desktop and vice versa), and this is something we want to expand and continue.
However, we do not believe in the "one interface that runs on both your desktop and your tablet". We believe in code reuse, in component-reuse (and, where beneficial, drop-in-replacement), compatibility and interoperability; but we also believe that a tablet interface and a desktop interface are not, and should not, be the same thing. The use cases and form factors are just too different.
We have no plans of bastardizing Plasma Desktop into a watered-down attempt at a tablet interface that also sort-of-makes-sense on a laptop. We feel this only produces interfaces that perform OK but not great on either kind of device. We want interfaces that work great on each sort of device. This is why we designed Plasma to be so flexible: we can afford to have different interfaces, and trivially keep them compatible with each other, without pouring gigantic amounts of resources on it.
So those who are concerned that we're going to do something nasty to the desktop interface: breath easy. We will continue to improve and work on new ideas on the desktop, as we did with Folder View and Activities, but we're also respectful of how people (including us) use our laptops and desktops.