See my above comment, but basically what fixed it for my (4S on AT&T) was to reset the network settings. AT&T said that sometimes after installation the update doesn't reset these settings even though it is supposed to.
I still get random usage overnight but that was always the case even when I had the 3GS on iOS4.
Thanks. We reset the settings to no avail. We gave up and got an iphone 5. Hope their fixes for the 5 are more timely since the discussion boards have reported the same problems on the 5...this will be our last chance with Apple. Not sure I am too keen about the way Apple is conducting business these days...
I have the same issue but only when im on roaming as im mostly always under wifi and dont notice it. I recently went aborad and my data plan skyrocketed to 800MB out of 1.3G monthly in 3 days despite being in wifi at the hotel. I bought my phone in Hong Kong and am using it in China with local carrier.
I must also say that since the unfortunate passing away of Steve that the phone and Itunes are seriously bugging. Small screen freezes, sudden app shutdowns, notes that doesnt allow you to add more are all examples of what is going wrong and isn't being adressed the way it used to be, i also now got an Iphone 5, but cant replicate it because for some reaon my 4S wont backup anymore despite installing all the updates, its driving me crazy, I cant pay that kind of money and then be let down like that anymore.
i also have a iphone 4s and with telus in august 2013 the phone under 5 minutes sent me 3 message on data usage you are at 65% you are at 85% you are at 95% it was connected to wifi to not usa cell data. and i got overcharged for some cell data that i did not use i tried to explain that to the cell phone compagny telus they would not listen, and told me to set the phone to not use cell data or to exchange it if i still get overcharged and they cannot do anything about it. so i think they have a bug to see your real usage. and it works to their advantage since they can overcharge you and blame you or the iphone as being the trouble. well i have no internet on the phone since 4 days since last update to ios7.0.4 so when i will see the data usage for those day it should read 0,will call telus to find out how to fix the phone if i do not get answer from here.