If I had to guess based on what we know so far about IOS 5 and what's rumoured about the iPhone 5, I'd say the battery life will be about the same or slightly better than the iPhone 4 currently has. My reasoning for this is (assuming this rumour is true), the iPhone 5 will have a bigger screen, thus warranting a slightly bigger case which would have room for a slightly better battery. But, this slightly better battery will need to provide a bit more power for the slightly bigger screen. Plus IOS 5 has more data centred features, thus using more battery. But, if the iPhone 5 ends up having the A5 processor, then it will have better power management. So really all I think it'll be about the same (which honestly wouldn't bother me since the iPhone 4 gets great battery life).