A dimmer screen will help prolong the life of your battery on the iPhone. You can adjust it by going to settings then brightness. If you turn on Auto-Brightness it will adjust the brightness automatically based on the lighting conditions you are in.
Yes it does and 3G on does as well . Hint! When charging the iPhone turn airplane mode on to charge faster . Airplane mode turned off all data and stuff like that it uses less battery life . Only do it when ur charging while sleep or when u know that ur not getting a call cuz u can't make or receive calls in airplane mode.