Just noticed my current Pay Monthly balance was showing a £12 charge for "data", despite the fact that this should be free. I rang O2 and queried it; they said it was for data roaming (i.e. use of data services when abroad). I do travel quite a lot and always have some voice roaming charges, but I NEVER turn the data roaming setting on. I explained this and the lady put me on hold while she spoke to the iPhone department. Eventually she came back and told me that there had been "problems" this month with some data being charged when it shouldn't be (I don't know if she meant roaming data or UK data charges), and I would be issued with a credit to cancel out the charge. O2 were very helpful and this has been resolved for me now, but it could be worth checking your own accounts to make sure you've not been billed for data.