Thanks Victor. It does help. But from what you state "as Nikon has defined it" Its not really AUTO, is it? AUTO means without user input. Its more of a Priority ISO with some built-in programming latitude. It will adjust the ISO (AUTO) in full MANUAL mode, but tend not to in Shutter, Aperture, or Program modes? Its more flexible in MANUAL than in PROGRAM? That makes no sense. It remains were you set it until it runs out of useable shutter speeds or aperture settings. Who defines useable? 1/4 sec isn't useable unless you are on a tripod and as far as I know, it doesn't detect its attached to a tripod.
Example: If the D100 is set to ISO 200, AUTO, P mode and I am trying to shoot fast moving objects and the light isn't so great, it may not give me a fast enough shutter speed. I would assume AUTO would bump it up to a faster ISO. But from what you said, it won't. It will just slow my shutter speed down. I know, I know. Its not a Point & Shoot. But even my N70 (no snide comments please) has a Speed Program for this.
Victor, my comments are not directed at you personally. Its just some fustration with the D100.