Looking over the LibHandy API is making me curious about HdyDialer. At casual glance it looks like it's basis isn't a generic gtk keypad, which in an OO sense is wrong, but might be "how things are done" in gtk. I don't really have the experience working with gtk.

An interesting thought, in an analysis of a graphical keypad, is where the extensible line gets drawn. i.e., does it stop at scientific calculator or does it go all the way to keyboard?