You can try to Install a Fake APN Profile.
Start your SAFARI via WIFI, go to http://www.unlockit.co.nz/
Install the Fake APN Profile and you will not have 3G/EDGE/GPRS accessing out from your iPhone.
To uninstall the Fake APN Profile, goto Settings > General and scroll down.
You'll see Profile - Fake APN.

This is good when you are traveling overseas.
You won't want your iPhone to access any data while you are elsewhere outside M'sia.
Hope it helped!

I'm using a Maxis subline with an iPhone not from Maxis.
I didn't want to use any data from my Maxis line hence i switched off the Data Roaming.
However, I found out my iPhone was STILL accessing data when my monthly bill came.
I suspected it was because of my Sygic GPS app.
From then till now, I'll install the Fake APN whenever I'm in Malaysia.