I have two i5's on my plan and one Droid. For the past two months my i5 Data usage has gone through the roof. I have spoken with tech support and they have walked my through all of the typical data burners and turned them all off. The i5's are burning 10x the usage of the Droid in the same environment. Is anyone else having this problem?
Go to http://www.apple.com and check their community help forum. There is a fix I believe for the data leak.
Some have posted you have to turn off iCloud to stop the leak. Additionally although a pain you can turn off cellular data when on wifi, and place the device in airplane mode when on wifi. There may be other solutions at the Apple site.