Thought I was just gonna chime in on this issue.
iPhone 4GS, currenly 6.0 (yet to update)
I got a text from my operator yesterday that I had exceeded the monthly limit for mobile data transfer (5GB). I was very confused and talked to them and they said that for instance on friday 2nd I had used 1.6 GB of mobile data transfer and just this saturday about 850 MB. I tried to recall what I had been doing but nothing came to mind and this just seemed absurd to me, in the 3½ years before I have never been over 2 GB in a whole month, now I had managed to use 5GB in 11 days.
Anyway, they agreed that it looked sketchy and gave me another GB to use and this morning I started monitoring and just like a lot of people allready have said in this thread it was 100% the new built in Podcaster app. I had it set to not sync subscriptions but to automatically download new episodes. However, the "Use cellular data" was off. Despite this, I started listening to a podcast this morning that I had allready downloaded and reset my usage to monitor. In just over 5 minutes I had 110 MB recieved data, only the Podcast app running. Note that all my subscriptions (only have four) should allready be fully downloaded and synced as I do this at home through WiFi.
I had to put the phone in Flight mode to stop it from downloading...whatever it is downloading. Note that whatever it is downloading isnt stored anywhere, the total data used on the phone hasnt grown at all, despite 100+ MB of fresh data was recieved. When I got to work I turned Flight mode off and it instantly resumed recieving data, it had downloaded 2 MB before I even had navigated to the usage display. When I killed the Podcast app, it stopped.
So in short: the Podcast app is running amok, downloading stuff (that it doesnt store in any way that can be seen at least). I called my carrier (Telia, Sweden) back this morning to inform them about this and also this thread because yesterday they said that they didnt know of anything that could cause this.