You may have heard the saying, "It's the journey, not the destination." I like this saying, but I have never applied it to user interface design.
Normally, when prototyping or sketching up a UI design, I focus on the end states, the screenshot. Once you have that, you link screenshot to screenshot in a kind of storyboard that shows the process the user will follow to accomplish a task. Most of the time, these are just paper sketches. Sometimes I'll do some high fidelity screenshots, but normally the sketches are enough.
What's amazing to me about the iPhone experience is how much time Apple spent, not on the end states, but the transitions through and to the end states. I'm really starting to think that what makes you feel so great about the device is just as much the easy to use screens, but the ease and smoothness of "getting there." The "journey" through the iPhone interface is filled with great transitions from beautiful state to beautiful state. It really is just as much about the transitions as it is about the destination.
I started to think about other great experiences that I've had at restaurants, shopping, even a few websites and I realized that a very large part of what made them so enjoyable was really the way they managed the transitions. Every step of the way was cared for, curated really. I've always known the "out of box experience" was important, and the functional flow through the application critical, but designing the visual flow through each functional transition, this is a whole new idea.