I just had this fantastic idea and want to get it out in public.
Apple should design an iPod that has a rechargeable battery that slides onto the back and locks in place. There could be more than one size (slim or fat), you could own several, you could have a dock / charger that could hold the iPod and a battery, charging both.
The problem of not being able to replace a dying battery would go away, and Apple could sell lots of batteries and chargers. There should be an option to charge the battery without the dock too.
For myself, I would not mind having a fatter battery on the back to extend playback time when I need it, or using the slimmer battery when size and weight are the important issue. There could even be one you load with AA or AAA batteries if you want.
What possibilities! Do you think this is a good idea?