The iPhone's finger-driven interface seems natural and obvious. But when you really think about it, making it seem that way was no easy task. There are no menus in the iPhone software, for example, and no checkboxes or radio buttons. Everything on the screen has to be big enough for a fleshy fingertip.
On the other hand, the finger makes an outstanding pointing device; heck, you've been pointing with it all your life. It's much faster to scroll diagonally with a fingertip, for example, than with fussy adjustments on two different scroll bars.
Here, then, are some of the iPhone's unadvertised taps, double-taps, and other shortcuts, all culled from iPhone: The Missing Manual.