I want to poke at this a little. I don't think the ribs on the handle of a knife qualify as skeuomorphism, since they serve a distinct functional purpose independent of their origin (namely, better grip).

I do agree that "leather for every app" starts to deviate away from the definition of a true skeuomorph. One could argue that the iPhone was trying to evoke the personal organizer of yesteryear, and therein lies the skeuomorphism, but since the functionality of an iPhone was so much broader than a personal organizer, it may be conceptual smear to call that skeuomorphism too (but I still think a good argument could be made that it is skeuomorphism, since the functional form of a prior design is still being evoked intentionally for aesthetic / emotional cueing purposes).