I'm getting my wife (yeah, sure, that's it) an iPod (either 15GB or mini) and I'm concerned that once the battery starts lowering its charge, a replacement might be costly -if at all ...

I've got a 3rd gen 40GB ipod which is not covered by Apple's manufacturer warranty anymore as it is over a year old & I didn't buy extended warranty. The battery barely manages to last over 2-3 hours.