Apple has historically made a big deal out of long battery life in its portable devices. I think, if the battery is going to remain relatively small, this must indicate that the device will be very efficient.
We are getting to the point with smartphones where higher specs - more cores, better GPUs, more vRAM, are starting to show minimal returns. The 4S is significantly more powerful than the 4 but, when the most popular apps are messaging services, skype, Tiny Wings and a bunch of note taking and organisational apps, the difference rarely shows up to the end user. Even when you do play high end games, does anyone feel that the graphics in games like Infinity Blade are poor? Does Real Racing 2 look ugly to you? The current hardware still has lots of potential - Real Racing 3 looks amazing and that will be running on the hardware currently in the 4S and 3rd Gen iPad. There's no need to ramp things up even more - after all, this is still a smartphone, not a home console or a laptop. People don't need quad core CPUs to read e-books, play Angry Birds and watch YouTube clips. But a slimmer device with good battery life? That's something that we can all appreciate.
I think Apple's new iPhone will eschew faster, bigger and more powerful and, instead, opt for slimmer, lighter and more efficient. If you're looking for an iPhone you can boast to your friends about and compare GeekBench scores, this isn't the phone for you. Wait, what am I thinking, people who boast about GeekBench scores don't have any friends!
If we assume the new iPhone will carry the smaller, 32nm A5 chip and will include the lower powered IGZO screen and the new generation LTE chipsets, then it's reasonable to think that the battery life on this device will be better than on the 4S while increasing performance a modest amount within a smaller, lighter product. Think about it this way, Apple refused to have LTE early on precisely because the battery life would be badly impacted. They made the same choice about 3G on the first iPhone. Do you really think that same company would go ahead and design a device with terrible battery life? That just doesn't make sense.