[Establish context: Mac OS X, "busy", custom cursors, animation, long-running processes, ...]
Jim Ingham writes: "About the spinning beach-ball: If your application is not fetching events from the window server after a second or so the system puts up the spinning beach ball. I don't think there's a way to tell it not to do this... So to avoid it you need to make sure that you pause whatever computation you are doing and spin the event loop every so often. That's probably a good idea anyway, since you want to give the user a chance to interrupt long-running operations if possible."