On windows xp, if you bring up the shutdown/logout/restart dialog and wait some 30 seconds, the background fades to black&white. I think it would be great if the UI would fade out like this under heavy load. This may be too resource consuming (it would imply repainting the desktop, and i have no idea how easy/hard that is) but it would be at the same time obvious to the user, and it would not annoy as much as a fullscreen throbber. It does not impede usage under load the way a throbber overlay would