I also believe it to be an iOS5 issue! Because my husband kept his iPhone 4 and I have the 4S...his battery is draining much faster and he isn't a heavy user during his work day, light usage of Twitter, Facebook, iMessage, and his battery is ready to be charged every night. Prior to iOS5 he could go a couple days with no problem.
I am a much heavier user and I definitely am not seeing stellar battery life, by the end of the day I'm squeaking by at 5-8% left at bedtime, I also could go a couple days if my usage wasn't as much..
Im not stressing, Im sure Apple is well aware and we'll have a fix soon! Im not going to turn my backlight down, turn off WiFi, etc, etc just to save a little battery life, that's not why I have this phone, so I can turn off half it's awesome features!
Lovin' my 4S!