I am so glad to see this, I have been trying to figure this out for days now. I bought my mom an iPhone 5. I walked her through Wifi setup over the phone. It was connected to her wifi network.
She's on a 2 gig plan so last week I checked her data usage to see where things were. Imagine my surprise to see 800+megs of usage just a couple hours after she had activated it. I am an android user who is not that familiar with IOS, so I didn't know where to tell her to even look over the phone. I had been going crazy trying to figure out how this could have happened...until this weekend when I was poking around on her phone in person. She had podcasts which totaled the amount of the data transfer, but when I went into settings for podcasts, the 'use cellular data' option was disabled.
It's pretty unlikely she'd use up all 2 gigs even with the podcast download, but I would be pretty pissed if this had been a shared data plan.