Hi vi_, the gpio exposes an interface to interact with via dbus. It means that it emits some signals and provide some methods to interact with. How you cope with that, depends on the language/tools you use. AFAIK both Qt and Gtk offers the chance to play with dbus (obviously). Python offers a native way to interact with dbus and there are Python bindings for Qt, too.
HTH your further researches
EDIT: did you find http://talk.maemo.org/showthread.php?t=62198 for example? And this one: https://bugs.maemo.org/show_bug.cgi?id=7070 useful, too...