Currently OS X uses the spinning rainbow pinwheel to indicate a busy system. It is often known as:
* The evil beachball
* The beachball of death
In Jaguar, it has been replaced with an Aqua-esqu translucent gumdrop with a vague pinwheel in the background. It is said that Apple has done such a good job optimizing jag that you will rarely see it.
I'll believe that when I don't see it... - Ross