With thanks to the new Retina display on the iPhone 4 announced by Apple earlier this week, the text and images will be more crisp, with the new device boasting a 960x640 screen which is 4 times the pixel size, as it's predecessor. However the issue arises, when users who use your app might notice that your graphics and text may not be as clear as they might have once thought.
For fonts, XCode will render them fine and improve the sharpness, but for graphics, you need to go back to Photoshop or whatever you guys use, clean up your graphics and make it look sexier. Icons, fonts written in as an image will have to be improved, otherwise they will look pixelated. It will be good practice, as you might also be targeting the iPad and for the iPad, bigger screen means you need higher quality graphics, so you could in a way target both devices.