Jürgen: Before we started to design, we analyzed literally every screen in all of Apple's apps for the iPhone. It was important for us to understand every nuance of Apple's user-interface "language." The amazing thing about Apple's own apps is the level of design consistency they achieved. They all speak the same language. This is so different from other platforms, and it means that your app can be a seamless part of the overall platform experience, too. When your app follows Apple's design conventions, users feel immediately at ease when they tap your app icon. It's a huge motivation to try to deeply understand Apple's design decisions down to every detail so you're able to do it the same way and create the same kind of experience.
One of the reasons the iPhone is so successful is that Apple really tried to make the technology behind it invisible. You don't have to think about how it works. I think that's the thing to aim for as a developer—make the interface essentially invisible, too. You take it out of your pocket and you just tap somewhere. You really don't want to make people think about where to tap or what's the next step. For users, the focus should be on the task and the goal, not the interface.
In iPhone design, beauty and elegance comes from a sense of simplicity. There are always many, many different ways that an application can approach a certain task, but only a few of those options will actually be simple and clear, something that people can easily understand and figure out. As developers, our first tries are often more complex than they should be. When you're finally able to do something in the simplest, clearest way, it clicks. It feels like the fog has lifted, and there's a sense of beauty in both the code and the interface.
Eye candy isn't as important to the beauty of an interface as a lot of designers think. That's not what sets you apart. If you figure out the right way to solve a user's problem and do it right, the eye candy is secondary. You can add some nice visual touches later, but it shouldn't be where you start. Even if you use only standard controls, the application will develop a personality and spirit of its own. If you put the right controls at the right place at the right time, then you have an application that's easy for people to use and accomplish what they want to do.
You don't need fancy graphics for that. We use standard iPhone controls whenever possible. Developers should think hard before reinventing the wheel and creating their own controls. People feel right at home with what Apple provides, and that's a great starting point.