The battery is a good candidate for a Do-It-Yourself (DIY) iDevice repair because Lithium-ion (Li-ion) batteries have a limited number of charge cycles and are therefore replaceable. Of course, Apple does not want customers to replace iDevice batteries themselves. Rather, Apple wants you to turn in your current iDevice for a new model, or pay for a battery replacement.
To that latter point, Apple has a published policy (http://is.gd/i773q6) concerning battery replacements. It works like this:
The default hardware warranty covers the battery for the first year of ownership
The AppleCare Protection Plan covers the battery ...