Sony has announced a new fuel-cell battery that runs on glucose. It works by breaking down carbohydrates with enzymes, in much the same way as us humans do. It doesn't seem to be ready for release yet, but the video after the jump shows that it's already capable of powering an MP3 player.
Sony are claiming 50mW per cell, so it's still not very powerful. Even the MP3 player requires a few devices daisy-chained together, so it will be some time before this is portable enough to be practical.