Since we're now using desktop files instead of arbitrary scripts or symlinks, it is pretty easy to make the list of autostarted applications editable through a simple user interface. And so here's
xfce4-autostart-editor is now part of
xfce4-session (trunk, rev 16232). I'm looking for an icon - or atleast an idea about how such an icon could look like - for this tool (unfortunately Francois is away).