One possible reason the iPhone 5 has such great battery life is the clever way the A6 changes its clock speed. While originally thought to be clocked at 1GHz, the chip has been clocked at 1.1GHz as well as 1.3GHz by Current Editorials.
While 9to5Mac saw the chip's speed drop as low as 550MHz, this seemed to do more with an as-yet-updated Geekbench app testing the chip (which also resulted in the 1GHz assumption). A Geekbench update seems to paint a different picture now. By all accounts it appears the chip can change "speeds," however, resulting in better battery life and dynamically tuning itself to the demand for CPU.