Apparently apple has already patented one of the things on my personal wish list for the iPad: a conductive stylus that transmits pressure and or tilt information. This type of a pen would basically turn the iPad into something like a Wacom Cintiq, at a fraction of the cost and with far better portability.
Up to now, drawing on the iPad has basically been fingerpainting, though several of the drawing apps have taken some novel approaches. Sketch Club, for example, can use your drawing velocity to control the line. Though there are styluses which you can use with the iPad now, none can support pressure or tilt sensitivity. Because of the way the iPad handles touch events and gestures, this has to be directly supported in iOS. While many of the drawing programs for the iPad are already pretty good, pressure sensitivity is a must for any serious professional drawing program, especially those that mimic traditional art materials such as charcoal.
This article goes into a lot more detail about their approach. This particular approach requires a special pen with a conductive disk tip that can trigger the touch panel sensors more effectively.