I wrote my first KDE4 plasmoid the other day. I can't release it because it's essentially a clone of something you aren't allowed to copy (maybe I can replace him with a penguin and release it that way though).
But I need to rewrite it first anyways, because I did it using the Ruby bindings for Qt4 and Plasma, and wow it's painful. It has a 50/50 shot of even initializing at any given point. When it does initialize, it has about a 1 on 8 chance of immediately crashing Plasma. And some things I just can't get to work at all, e.g. setting a default size or resizing the applet programmatically;
X-Plasma-DefaultSize in the metadata is supposed to do it but it does nothing. And it's not just my system (using KDE 4.3), because I tried it on a Kubuntu machine using stable KDE 4.2 and had the same problems.
The other snag is that the documentation of the Plasma API is buried so deep on the KDE site that I don't even know how I found it. Here it is for those who care (and for my own future reference). I hit lots of dead links on the KDE site on the way there.
Next step is to rewrite the plasmoid in Python or C++ I guess.