If you can't use your hand/arms and need a speaking device then the options are reduced. I have a free program on my netbook called CameraMouse (just google it) that uses the built in webcam to move the cursor around by telling the program to focus on a part of my face. When I move my head the cursor moves and I can select letters on an onscreen keyboard or through another interface program like Dasher. Joel has more experience with this than me because I can still type and haven't practiced.
Other than that there is the Dynavox EyeGaze system which I have no experience with at all.
I don't know of any apps for the iPod/iPhone/iPad for someone who can't type but things change so fast now that you never know what they will come up with next.
