AA battery powered mp3 player?

How about an mp3 player that uses a popular cell phone battery? Some of those battery models retail on the net for only around $5 or so each, and are easy to find. They seem to be 700 mah or more and 3.7 v, or at least 30% more capacity than the battery in the Fuze. I want to be able to carry spare batteries, and swap the battery quickly when it is out of power.