How to turn off data on iPhone since recent iTunes carrier setting update?

Question

In the past few days my iphone has been using 3G for data and I have no control over it. At home it is connected to my wifi internet which has always been the case and which is fine but when I leave home it turns on data via 3G and there is no way off easily turning it off short of going into aeroplane mode or turning phone off altogether. I Only became alerted when I got an sms telling me my balance was low on my prepaid (with Telstra) when I knew I should have had plenty of money left on balance. Lucky I've only had about $20 used - how am I to know the phone in my pocket has been always connected. The problem has arisen since a carrier setting update a few days ago via Itunes. I can turn off 3G but then the phone just reverts to 2G and the data keeps flowing !
There is no where to alter the cellular data network or APN settings since I installed a 3G sim .... previously with a 2G SIM you could enter cellular data network menu and enter APN username and password but these have both gone.
This is a major problem and could be very costly if someone does not realise it quickly .
Any solutions .... will speak to telco later today but they will inevitably deny an responsibility as they do !

Answer 1

Unfortunately this may be contrary to the terms of the agreement that I have just agreed to so as to be able to participate in this thread but I consider the inability to limit the data access, to provide the owner with the option to explicitly authorise data access requested by an application as not just "bill shock" but THEFT. We require some consumer advocacy.
I am a prepaid client who only wishes to use the data based apps when wifi is available. The workaround that worked until two days ago was amending the APN but now, on an iPhone purchased at the Apple store in Sydney and presumably an officially unlocked phone, no longer displays the APN option. Despite being very aware of the vulnerable state of the phone and the need to avoid any data based application I yesterday I still managed to incur two unwanted data charges of $18 and $9. I consider that THEFT. Even on a data plan I would be unable to satisfactorily control data usage.
When the unwanted data exhausts my current prepaid balance, as it has several times in the past, I will have to convert my iPhone into an iPod Touch and obtain another phone. I certainly will investigate whether Android permits controlling data access.
I am very disappointed as I believe the range of apps provides a compelling argument for the iPhone but the customer's inability to control data usage is unacceptable.