>Can someone explain the logic why the Auto ISO function is >not part of the standard ISO function? It could have easily >been inserted as ...800, 1600, H-1, H-2, AUTO, 200...
Because Auto ISO, as Nikon has defined it, is based on you first picking SOME value to work from. If Auto ISO were a selection from the ISO list, then you would have no defined starting point.
>Also, when I use Auto ISO, the D-100 always seems to use the >ISO that was chosen. Am I missing something here?
In S,A, and P mode it will stick to the chosen ISO until it runs out of useable shutter speeds, aperture settings, or (in the case of P mode) both. At that time, if there is latitude remaining in the necessary direction, the camera will adjust the ISO to give you a correct exposure.
In M mode, I have noticed that you can set a shutter speed and aperture and the camera will vary the ISO over the range of 200-1600 to get a proper exposure. I call it "EV priority".