When I start the X11.app then it launches an xterm automatically. I've long since switched from the "ordinary" xterm to rxvt-unicode so the initial xterm just hangs around like a wallflower never being used. Whilst it's not particularly bad, it spoils the look of the place so I'd like to stop it launching. How do I do it?
in a Terminal. However, note that there really shouldn't be necessary to every start XQuartz.app (or X11.app; see the comments) manually. It will start automatically when you run an X11 client, and then it will not start its defaults startup application.