As I mentioned in AJAX & Interface Design, users may not notice very fast and incremental (often affecting only a small portion of the UI) updates to a Web page because they are used to seeing full-page rewrites in response to their actions. Motion (to which we all have an immediate instinctual response) can be used effectively to bring attention to areas of change.
The Apple UI makes use of animation to highlight change. It’s useful to note that the animation appears right where the user originally initiated an action. For this reason the small spinning icon may be enough to communicate that something is happening. If that change were to happen elsewhere on the screen however, a stronger indication of change might be necessary.