It's probably a reality if a developer wanted to make it one. It would probably be a sizeable application, but would prove very useful. I know that when I toy around with a few on my computer, they just don't seem that useful unless I was somehow disabled.
However, on a mobile device it could prove more useful due to the fact that you don't need some big clunky machine in front of you. The mobile device (G1 at this point), could almost act as a personal memo machine, and could be scaled all the way up to a full on text-by-voice entry machine.