Despite competitors having talking points about how the iPhone battery isn't replaceable, that's not entirely the case. It's true that the user isn't able to buy a battery and swap it out on the road as they can with many existing phones, but Apple can do it for you, for a fee of $85.95 plus applicable taxes:
"If your iPhone requires service only because the battery's ability to hold an electrical charge has diminished, Apple will repair your iPhone for a service fee of $79, plus $6.95 shipping. Be sure to follow these instructions for optimizing battery life and troubleshooting battery charging issues before submitting your iPhone for battery replacement."
Other than the cost and having to go about 3 business days without a phone, there is one other possible downside to the procedure: you lose all your data. The moral of the story is to remember to synch often.