Unfortunately, like the iPod, the iPhone 3G features a built-in battery that is not intended to be user-replaceable. The iPhone's battery is stated to be capable of providing up to seven hours of video, six hours of web browsing, or eight hours of talk time (depending on configuration). Music playing is stated to have a 24 hour life. The iPhone's battery will also allow for up to 300 hours of standby time.
Below are the specs on the battery if of iPhone 3G: