According to headset-button-enabler description, when it is installed and a headset button is pressed, "ButtonPressed" signal is emitted by HAL... it should be possible to catch that signal in Python somehow... I cannot help with Python, as my Python skills are weak, but have a look at headset-control package sources, especially at two files: headset-controld.c and headset-control-obj.c. If you understand C language, you will notice this isn't rocket science, and it requires only Python glib, dbus and hal bindings (maybe even one of these is not needed, I skimmed through the code now very quickly while I'm on the train), which, afaik, are all available for Python on Maemo. So, in my oppinion, package headset-control could be used as a reference implementation, if someone has more time, (s)he can write a wiki page basing on this code.