Like iPods, the iPhone's battery is not easily replaceable, but there are many online guides that show how it can be done. During the warranty period, either standard or extended via AppleCare, Apple will replace a malfunctioning battery gratis. Outside the warranty period, replacement is $79, plus $6.95 shipping. The program cost is $85.95 per unit. All fees are in US dollars and are subject to local tax.

More About iPhone Batteries from Apple

MacWorld's iPhone Battery Article

The typically cited figure is that the LiON battery will lose 20% of it's capacity after 400 charge cycles. So, after a year of daily charging, you should still have 80% recharge capacity.