I have the same issue too. After upgrading to 6.1.2 my iPhone 4s is CONSTANTLY downloading data. Everything - and I mean everything is turned off. I have been researching this for days, following every suggestion I can find. To no avail. I can watch my data usage tick .01MB PER SECOND. Yes, PER SECOND. I am not using iCloud for anything. Turned off iTunes, don't have Podcaster installed.
I used 1.04 GB yesterday in one 24 hour period.
What is fascinating, and truly dispiriting, is that I have seen complaints about this from all over the world. It seems that there must be hundreds of thousands of people who are affected by this issue - and there is no help to be found.
It is not an option to 'turn off cellular data when not using it'. The entire purpose of the phone is to be able to receive this data when you need it. Many of my most used apps are rendered useless if the phone needs to be used in this way. And STILL, this solution does not solve the problem - the moment the phone enters a wifi area it starts gulping down data for no reason at all.
I hope help comes for this soon. I have been a dedicated iPhone user since the day the 3GS came out. I love the products - but this will force me to, rather unhappily, go to another provider.