For a long time you may designed for the web using Photoshop, Fireworks or illustrator and developed skill to get work done rapidly. Since 2007 Apple has introduced iPhone and let developers to create applications for their clients. In compare to desktop size of iPhone is too small, plus it lacks mouse and keyboard used traditionally. This is little bit challenging for you because of its hardware and software. You have to consider all factors while you are going to iPhone App UI design. The rules and regulations of desktop UI design will not work in iPhone UI design.
For sake of simplicity and design convenience we can divide app types into five: ‘serious tool’, ‘fun tool’, ‘serious entertainment’, ‘fun entertainment’ and ‘utility’. Each requires their unique user interface decisions.
Serious tools always use a minimal color palette and are driven by data rather than graphics. For instance, Mail, Dropbox and Instapaper. Study the UI of these examples you will quickly get clues that how a serious tools should be presented.
Fun tools are mean to encourage leisurely productivity therefore there are moderate use of color and graphics and are always fairly simplistic in hierarchy. For instance, The Ocado app and Where To?
Fun entertainment includes games so they are always graphic rich app and use of sound and appropriate visual feedbacks are necessary elements. If user has sound turned off at this juncture you should provide some kind of visual feedback.
Serious entertainment is apps that use standard controls top and bottom but they contain custom content mean to be entertaining. For instance Sky News app, Movie Now and iTunes app.
In short, for best iPhone app UI designing consider above given elements during designing.