The iPhone 4 introduced a high resolution display with twice the pixels of previous iPhones. However you don't have to modify your code to support high-res displays; the coordinate system goes by points rather than pixels, and the dimensions in points of the screen and all UI elements remain the same.
iOS 4 supports high resolution displays (like the iPhone 4 display) via the scale property on UIScreen, UIView, UIImage, and CALayer classes. If the object is displaying high-res content, its scale property is set to 2.0. Otherwise it defaults to 1.0.
All you need to do to support high-res displays is to provide @2x versions of the images in your project. See the checklist for updating to iOS4 or Apple documentation for Supporting High Resolution Screens for more info.
Adjusting Sizes
Click here to see how to adjust View Frames and Bounds .
Additional References
Apple Documentation: Points vs. Pixels
Apple Documentation: UIBarButtonItem Class Reference says "Typically, the size of a toolbar and navigation bar image is 20 x 20 points."
Apple Documentation: UITabBarItem Class Reference says "The size of an tab bar image is typically 30 x 30 points."
