I am running into the same problem. i just bought this iphone 7 days ago and it has used 1.1GB already. I just checked my bill and it is showing that I used over 200,000kb during a time my phone was not in used. I need help. This appears to be a tactic to get customer angry to break contract and charge us the $350.00 termination fee. Please help.
Run your phone on wifi whenever possible. Go to settings/General/Cellular and turn off "Use Cellular Data" for iCloud Documents, Itunes, Passbook Updates and Reading List. You can also turn off Cellular Data for everything else (mail, browsing the web, streaming Pandora, etc) by turning Cellular Data off entirely. These things will run on WiFi instead (assuming you have connected your phone to wifi). You can always turn cellular data back on when you are out and about and not on wifi.
That being said, there is a known issue with cellular data on iphone models running iOS 6. This article only talks about the iphone 5, but I suspect it is a problem on other models running iOS6 too. See if what this article describes sounds familiar.