OK, rebhana, it isn't anything broken with dbus-switchboard. You just have to add lines to the two mimetype files.

First, /usr/share/applications/defaults.list

application/postscript=hildon-dbus-switchboard.desktop

Next, /usr/share/applications/uri-action-defaults.list

application-postscript=hildon-dbus-switchboard.desktop

Note that it is application-postscript not application/postscript in the uri-action-defaults.list file. And no, don't ask me why.

I will add this type to my postinst script, might as well make it available for everyone.