As Simon Edwards already noted, thanks to me and him (mostly him, I have to say) starting to work on Plasma tutorials at the same time, now KDE's Techbase has a nice set of Python Plasma tutorials. They cover creation of applets and connection to DataEngines (see the Plasma page on Techbase for more information). I plan on starting a tutorial on writing Plasma DataEngines on Thursday, if time permits.

For anyone wanting to try these out: you need KDE 4.2 (currently RC 1), PyKDE4 and kdebase compiled with Python support (not the case of openSUSE at the moment).