How to stop an iPhone 3G from using cell data, ever? My wife has inherited my iPhone 3G and it's using a few megabytes of GPRS data every month even though she doesn't deliberately use any services that cost mobile data. The problem is that she doesn't have a data plan, so these few MB's cost as much as a few gigabytes would with a data plan. 

1: How can I set up the iPhone 3G (OS 3.1.3) so that it absolutely never sends or receives any data except on wifi?

2: Also, what is sending/receiving this GPRS data if it isn't the user? My wife never launches the web browser or other Internet-needing apps.

Please don't simply suggest the obvious get a data plan because my wife doesn't want one. She wants to use the thing as a cell phone and for text messaging, but certainly absolutely positively no Internet or e-mail or whatever, except when she's on our home's wifi.

I thought I had thoroughly turned off mobile data. Here's what I've done so far (written from my memory, might be inaccurate):

Settings > General > Network:
- enable 3G = off
- cellular data = off
- data roaming = off

SBsettings:
- 3G = off
- data = off
- bluetooth = off
- location = off

Based on all this, the damn thing should only use wifi, but somehow it uses GPRS data anyway. Argh!

Solution:
Well, I didn't manage to turn off the mobile data usage despite all your very useful and helpful suggestions. Instead, I spoke with the provider and they offered me 8 months of 1GB-per-months for free, and after that I'm free to cancel it or continue at €7 per month (or whatever the rate will be at that time). They also gave me the money back for the data costs on the last bill. Very good customer service, thank you tele.ring of Austria.
In my country (Australia) we are able to ask the mobile provider to disable data on our accounts to ensure we cannot use any at all, and thus cannot be charged for it unwillingly. It might be worth calling up the provider and asking if they can disable the data service.