Eric Litman recently tweeted about the Taking a Peek: How People Really Use the iPhone by Create With Context.

It summarizes a UI/UX study performed on iPhone applications with a people of varying iPhone expertise. It is very eye opening and a must read if you are working on the interface for an iPhone application.

Some items of interest.

• Icons don't always mean the same thing to people such as the magnifying glass to search vs zoom and the x button to clear vs as a "go" button
• Sometimes users expect direct manipulation (tapping on something) vs tapping the edit button.
• Selecting an item vs getting information about it can be confusing.
• Sufficient highlighting and feedback are critical.
• The importance of the icon when purchasing

And many more. You really need to read it.

And, I'm one of those that think every application should swipe to the next item/document and go into landscape mode for easier reading.