Apple has confirmed that a 'very limited' number of iPhone 5s handsets suffer from a manufacturing flaw that causes reduced battery life. According to Apple's Teresa Brewer, who spoke to Brian X Chen of the New York Times:
We recently discovered a manufacturing issue affecting a very limited number of iPhone 5S devices that could cause the battery to take longer to charge or result in reduced battery life. We are reaching out to customers with affected phones and will provide them with a replacement phone.
Chen points out that the flaw is not related to a defective battery. If you're having the problem, and/or if Apple reaches out to you about it, let us know!