Maliit and Plasma Active
Until now Plasma active had an on screen keyboard that served us well, but due to its implementation had some limits that couldn't be easily overcome.
However there is a Qt based virtual keyboard project that is very promising: Maliit
It's the one that was used in the Nokia N9, and already made a good progress since the version used there.
To have a good user experience on Plasma Active it should be well integrated, both as in look and behavior with the rest of the environment. Luckily Maliit is transitioning itself as well to the use of QML to write the user interface, making very easy to switch to a platform specific UI while all the logic stays untouched (I must say I'm quite impressed by this input method framework).
This is the result of just some hours of work towards a keyboard interface that uses Plasma UI components, and hopefully it will be in the next Plasma Active release.