Anand Shimpi in his epic iPhone 5 review (highlights are mine):
The reality is that with more performance comes increased device usage - in other words, it’s highly unlikely that with a 50% gain in performance users are simply going to continue to use their smartphone the same way as they would a slower device. Usage (and thus workload) doesn’t remain constant, it’s somewhat related to response time.
Where things get complicated is if you use the faster CPU, GPU and network connectivity to increase your usage of the device. In that case you could see no improvement or even a regression in battery life.
This fact is easily forgotten when we talk about new devices and the expectations in battery life.