The iPhone 4S was released last week with iPhone 4S plans starting at $199 with each of the carriers. During the presentation, something was revealed which seemed a bit unusual. The iPhone 4S standby time has dropped 100 hours.
The technical details of the phone indicates that the iPhone 4S standby time is 200 hours which is still a very long time. However, the iPhone 4 was touted as having a standby time of 250 hours and going back even further to the original iPhone, that had a standby time of 300 hours.
In comparison, the Samsung Galaxy S II has a standby time as 710 hours on 2G reception and 610 hours standby on 3G reception. It is difficult to compare the two in a real world environment however as both likely do their tests differently.
The most likely reason fro the drop in the standby power usage is the introduction of the dual core A5 CPU. It looks like they may have traded off some speed for battery life.