I've just returned from holiday and used my iphone for wifi networks, i came home to a data bill that i've queried, apparently the iphone automatically switches the data roaming on if the wifi drops, i've thought i've been on wifi but the phones switched itself over to roaming and accrued £27 of data charges!!
The only way to alleviate this is to keep the data roaming off and switch airplane mode on when using wifi!
If you incur any such charges, phone o2 and speak to the iphone faults department, they will help you out by refunding you after they look into your account... seems to be a common unknown fault!!
Hope this helps someone else!