Preview at WWDC likely in June, followed by September launch.
iPhone 4S Battery Life Worse for 3G Data and Media as Compared to iPhone 4
For tests with 3G Data, Audio Playback, Video Playback, and Video Recording, iLounge found that the iPhone 4S came in under the iPhone 4's battery life times. For Wi-Fi and FaceTime testing they found it to be similiar to the iPhone 4, and on Cellular Calls, they found the iPhone 4S slightly better.
For a detailed and complete list of data see iLounge's review. This graph compiles some of the data (AT&T only):
One interesting finding with 3G data was that while their Verizon 4S unit ran for the same time as the AT&T model (~ 5 hours and 54 minutes), they found the Sprint model to lag at 5 hours and 23 minutes. Comparisons across mobile providers may not be completely fair due to differences in signal strength which could impact battery life.
In voice calls, they found the AT&T iPhone 4 and AT&T iPhone 4S to have comparable times of around 7 hours and 6-16 minutes. The Verizon 4S model, however, showed a surprising 8 hours and 27 minutes, while the Sprint 4S again lagged at 6 hours and 27 minutes. iLounge notes that the Verizon model may have shown 4 bars more often than the other two.
Both media playback and recording has taken a hit on the iPhone 4S as compared to the iPhone 4. For video recording, of course, it should be noted that the iPhone 4S is recording much higher resolution video than the iPhone 4.
Ultimately, their conclusion on iPhone 4S battery life is based on what your exact usage model is:
If you spend much of your day near a Wi-Fi network and only rely upon the iPhone 4S for web browsing and phone calling, you won’t notice a major difference—unless you’re thinking of switching to Sprint, in which case we’d be a little concerned. Similarly, if you use 3G data, plan to record or play videos, or want to listen to hours of audio during the day, you can expect greater battery drain from the iPhone 4S.As we mentioned, there may be some confounding data due to regional signal strengths, but given the other complaints about Sprint's data speeds, it still seems to be the worst choice amongst the iPhone carriers. iLounge's full review covers many other areas and is worth a read.
Now, we should note that some users seem to have been experiencing dramatically decreased battery life on their iPhone 4S. One solution to that is said to be the calibration of the battery, in which case you drain your entire battery once and then charge to full. Some have found their battery gauge to be inaccurate until this is performed. Apple includes other tips on their site about improving your battery life on the iPhone.