I don’t want an iPad because I don’t want to carry around a big device just to read.  I want to read on my iPhone.  With one hand.  (Settle down now.  I need the other hand to hold a glass of wine.)  But the iPhone has a small screen.  Sure I can zoom, but that requires finger gestures and also scrolling to pan around.  Tradeoff?  Maybe not so much:

Imagine a box.  Laying on the bottom of the box is a piece of paper which you want to read. The box is closed, but there is an iPhone sized opening on the top of the box.  So if you look through the opening you can see part of the paper.  (There is light inside the box, don’t get picky on me here.)

Now imagine that you can slide the opening around the top of the box so that even if you can only see an iPhone sized subset of the paper, you could move that “window” around and see any part of the paper.  You could start at the top left of the box and move left to right and then back to the left side and read.

Suppose you can raise and lower the lid of the box so you have two dimensions of control.  You can zoom in and out, and you can pan the iPhone-sized-opening around.

Now, forget about the box.  The iPhone has an accelerometer.  It can sense when you move it around.  With software it can perfectly simulate that experience.  I can read anything on my iPhone with text as large as I wish, without scrolling, by just moving the phone around.  With one hand.

This should be the main UI metaphor for the whole iPhone OS.