Haven't needed to replace the battery but I've popped apart my ipod for diagnostic reasons before.. very easy to do and all you need is an old credit card or hotel card key or something..
what you do is feel about the ipod so that you can make a small gap to stick the card in. Since the ipod is clipped on and not necessarily perfect it shouldn't be hard to negotiate a little bit of a nudge in the backing.. err what you do is firmly hold the front and back so that you can try and create a gap sideways to stick the card in...
This is fairly hard to describe, but then you'd stick the card in and run it around to pop the backing loose, getting the card to go around the edges it a bitch so be careful but it's not impossible.
After that you'll have the backing loose but it probably won't pop off. Stick the card in the gap and try to get it so that the hard drive and battery aren't snagging on the backing. It should come off pretty easily after that.
The rest is cake, swap battery, snap back on, and charge
