Many photographers already use a tablet like the Wacom Intuos to work in Photoshop. You would think that an iPad would be even better because you can touch the very screen that you work is on but it's not that simple. Due to a number of issues like lack of pressure control and the fact that your hand itself will "select" parts of the screen, the iPad never took off as a professional tool. That is until I saw this video.
Brian Yap is a professional illustrator and by using Adobe Ideas for the iPad and a Nomad Compose Stylus, he makes illustrating on the iPad look easy. If Apple eventually decides to create a stylus that can compute pressure, the iPad could be a game changer in the photo retouching field.