Whenever I fall asleep when listening to something, the interesting part is the next morning, when I try to reconstruct where I left off so I can pick up again. Falling asleep doesn't feel like a discontinuous process, but the audio recording reveals otherwise - when I try to pick up where I left off, I always find that there are moments that I remember distinctly separated by stretches that I have no memory of at all. I have to be careful to go back not just to the last scene that I remember, but the last scene that I remember and can explain how the characters got there and why.
I can imagine an iPod of the future that would actually detect when you fell asleep and shut itself off. Much better than the sleep timer.