This happened to one user in our family plan after he updated his 4S with iOS6.
Massive outgoing data charges caused him to use over 70% of his 2GB in just 5 days. It shows this data being used at night, but really this is how ATT clumps together all the usage at the end of the day--it is not real time, as ATT confirmed for me.
It is all data that is being SENT. So, that leads me to believe there is a bad bug in the update that is causing some endless loop of transmission. There is no app yet that will show you exactly which apps/programs are sending the data. Nothing else on the phone changed--all the settings for data blah blah blah were unchanged. Before this iOS update, this phone used only half of the 2GB allowance monthly. Now it is on track to suddenly use that within a week!!!
ATT acted like they are surprised this can happen if you are on wifi. Well, I've also learned today that when your phone idles in order to save battery use, it disconnects from wifi and goes to 3G. Short term solution is to plug it in. Also turn off the cellular data use when not needed. This is not the long term answer. Apple needs to fix the software and or my son's phone.
Incidentally, another user on our 5 phone plan also updated to iOS6 last week but does not have any change to data use. The rest of us are holding out on the software update until this is fixed. I still think it has to be something that went wrong with my son's download. Even spyware would be more subtle than that!
He can't get an appointment until tomorrow at Apple. In the meantime, staying plugged in and on wifi. Under iCloud/Documents & Data, make sure use cellular is off for iCloud storing. I hope someone finds, and shares, the answers.