iOS 4, Disable Data Connections http://snapvoip.blogspot.com/ iOS 4 Tips
If you have an iPhone and no data plan, then it is better ti turn off your data communications save you some money. iPhone will automatically try to connect to the net via any available medium. Once you disable the cellular data, it will only connect via WiFi Connection.
Just tap on settings -> General -> Network and turn off the Cellular Data and turn off.
If you do not do this, without a data plan, you might end up with a hefty bill from your carrier.