Although only in development, Toshiba showed off this new tech, designed to give either 100mW or 300mW power output for upwards of 60 hours on a single cell. In this case, mp3 player can refer to any small electronic device, from cell phones to portable DVD players to... maybe even laptops. Fuel Cell technology uses liquid fuel, in this case Methanol, mixed with water, to produce power. Toshiba has developed a passive system that gets over some of hurdles associated with fuel cell tech, such as needing a very diluted mix and large tanks. These cells use nearly pure methanol, and the smaller of them can supposedly give 35 hours of media playtime on a minuscule 3.5mL of liquid.
This is a neat idea, and having an mp3 player that could last for 3 days in my palm is certainly amazing, but I would wonder how many people would want the hassle of actually having to "fill up" their mp3 player as opposed to plugging it in to a wall somewhere. Might make a great outdoors style thing, though.