MacRumors has been pointed to a slew of possible iPhone 5 components, including the battery inside the iPhone 5. According to them, the iPhone 5 battery is pretty much the same shape as the iPhone 4 and packs the same power.
Specifically, the purported iPhone 5 battery is 3.7v and 5.3Whr while the iPhone 4 battery is 3.7v and 5.25Whr. The iPhone 5 battery was spotted by MacRumors sporting the "DVT_B3" labelling which suggests it's a late prototype. Perhaps power management will be different in iOS 5 but it looks like the iPhone 5 will have a similar battery life to the iPhone 4. [MacRumors]