The updated platform uses a higher resolution screen (960×640 as compared to 480×320). This additional resolution also means that any graphic elements need to be designed twice. One set of images for the lower resolution and optional high resolution set for iPhone 4. iOS4 still considers itself 320×480, but introduces a scale factor you can check or provide upscaled graphic assets for it to automatically use. In other words, the iOS4 requires low resolution artwork and can scale it up, but you can’t expect to simply build out a high resolution version and have the OS scale it down.
We’re currently working on updating our iPhone GUI PSD to reflect the forthcoming iOS4 update. We’ll likely release 2 new PSDs. One for 480×320 and another for 960×640. We’re going to try and make everything as pixel perfect as we can while maintaining assets as shape elements for easy scaling. We’ll be sure to post an update when it’s done.