If you do not want internet access on your iPhone, and would like to block it all together, there are two ways to do this. If you are using iOS4, you can go to Settings->General->Network and turn off the Use the Cellular Data option to stop internet access.
If you are on an older OS, simply point your phone’s browser to unlockit.co.nz. I have used this several times, and it doesn’t always seem to load properly. If you get an error, or a search page, just cancel that page, and try again. Here is the home page.
Press the Continue button to go to the next screen.
Select Disable Data (Fake APN), and follow the instructions. This applies a random APN to your phone, so when it tries to log onto the web it is not able to. This can be removed at any time by going into Settings->General->Profiles, and choose to remove the fake profile.
If you would like to activate internet access on an unlocked iPhone on a carrier other than ATT, check out my blog post coming soon on that.