I have thought of using my Sony while on the treadmill, exercise bike, or elliptical machine and I decided not to do it. I don't want to get sweat all over it, I don't want to accidentally drop it, and trying to hold my head somewhat still in order to focus on the words just sounds hard.
Someone should invent some glasses or goggles that one could wear that would display the words for those of us with the worst cases of reading addiction.