[aiharkness]

BlackBerrys do not have a setting to turn off only mobile network data. You can turn off data, but you will turn off all data, which means wifi as well as mobile network.

The only way to guarantee with certainty that all data is over wifi, you will have to turn off mobile network when using data apps. Be aware this will turn off voice unless you have UMA.

You will also want to make sure all your aftermarket apps are really closed and not running in the background when you are away from wifi and the mobile network is on.