Ive seen a couple of posts on this below which were dismissed in quite a patronising way, but just wanted to note that despite the "only use wifi" option being selected on the app, the app can still very much use 3G!
I use the app to fall asleep to - last night the Phone reverted to 3G, which the app used and will now cost me around £20 in data charges.
This has happened a number of times with the app - completely my fault for not remembering to turn off 3G in my network setting when I know this problem - but please know that your app can still use 3G despite the wifi option being selected. I have a 4s on iOS 5.1.1 - tv catch up was literally the only app running and I know that this has only ever happened on nights I use the app and forget to turn 3G off. And yes, the option for wifi only was definitely selected.
Please look into this or add a warning - not fair!