PyQt...
Update: "hgg" writes to tell me that my book is still available at http://www.commandprompt.com/community/pyqt, which I didn't know. Which means that my offer of sending out a pdf file isn't necessary anymore.
Seems I was five years too early for once in my life. Python as Visual Basic for the Free Software world was exactly my pitch when looking for a publisher for my book on application coding with Python and PyQt. The book is hard to get nowadays: the publisher has let the online copy disappear and paper copies are exceedingly rare. I periodically try to interest the publisher in a scheme where someone else can update the text, but that has never gone anywhere. In any case, I can send interested people the pdf file, I guess. More than 600 pages...
Personally, I've never cared for Ruby, and while I admire Korundum, I've never seen the problems with PyQt and PyKDE. Actively maintained, stable, easy to use (every method can be a slot!), integrate well with KDE, maintainer is a great guy, nice mailing list. In the early days the biggest problem were that there were no distributions carrying the binding; nowadays it's one of those technologies that may not be completely up-to-date in the buzz department, but that just work. PyQt and PyKDE work fine. You can depend on them.
A newer development is Kross, in KOffice. Basically Kross is a bridge between a set of classes you define to be the scripting API of your application and a language interpreter. All languages use the same API: Kross has pluggable interpreters. Currently Python and Ruby, but people seem interested in adding a Javascript interpreter. Now I've just had the pleasure of hacking on a Javascript application a little, but that's a whole other rant. There are mature Kross bindings for Krita and Kexi and an experimental one for KSpread. Personally, I'm interested in doing one for KWord. Sebastian Sauer is now working on a macro system, too.
Kross is a little underdocumented -- like everything -- but Cyrille Berger has been writing a tutorial for people who want to use Kross in their application. It's not finished, but it has enabled Isaac Clerencia to do the KSpread plugin.
As a side-node: a current problem with application scripting seems to be that we don't have a good idea of what a KOffice document tree should look like from a scripting point of view. Something I need to think over.