Despite what some people claim, KDE Plasma's development is a huge success. From day one Plasma was intended to be extremely flexible. The first manifestation of the Plasma technology was the traditional-looking Plasma Desktop.
Despite its – at first glance – traditional interface, it featured some unique design decisions, like the ability to reserve only a certain area for icons. The next step for KDE was to develop an interface specifically targeted towards netbooks.
Plasma Netbook's "Search and Launch" page
Although it behaves rather differently, most code is shared with Plasma Desktop – improvements in the foundation technology are shared with all Plasma implementations. Plasma Netbook's design is centered around the concept of pages. By default there are two pages: a Newspaper page that displays current news that is fetched from RSS sources, the weather forecast, etc., and a "Search and Launch" page for finding and opening applications and documents.
While Plasma Netbook reached production quality with KDE SC 4.4, a third shell entered into development. Plasma Mobile is geared towards smartphones and similar devices. With it came improvements in finger friendliness to the whole Plasma framework.
Early prototype of Plasma Mobile (click for video in .ogv)
Now KDE's Plasma developers merged concepts of Plasma Mobile with the Plasma Netbook's Newspaper concept and the result is Plasma Tablet. While it's still experimental (just like Plasma Mobile is), it shows an additional use case for the technology without the requirement to recode everything from scratch.
Plasma Tablet on a device that uses VESA drivers and therefor doesn't run at full performance.
According to an e-mail on the KDE Plasma mailing list an unnamed manufacturer already plans to use a Plasma-based shell on upcoming tablets. Along with the port of KWin to OpenGL ES, the future for KDE technologies on mobile platforms certainly looks bright.