When does Apple consider the battery defective for purposes of warranty replacement?
Apple’s Battery Replacement page says, “Your one-year warranty includes replacement coverage for a defective battery. You can extend your coverage to two years from the date of your iPhone purchase with the AppleCare Protection Plan for iPhone, which is expected to be available in summer 2007. During the plan’s coverage period, Apple will replace the battery if it drops below 50% of its original capacity. If it is out of warranty, Apple offers a battery replacement for $79, plus $6.95 shipping, subject to local tax. Apple disposes of your battery in an environmentally friendly manner.