Maybe the most important idea and the success factor of iPhone and iOS in general that you interact with the device using your bare fingers, no need to stylus or any thing.
I recommend you watch Steve Jobs keynote when he launched the iPhone for the first time.
I'm not an expert but I think you can implement your requirement using custom gestures.
Hey you can do it by getting the touch event of finger and by getting the coordinates you can draw signature on the screen and you can also store them. Their is many other ways to do it. no need of any pen at all.
Trying to win the world.....................
SCJP 6.0 , SCWCD/OCPJWCD 5, BlackBerry Developer