On ATT, and I assume on other carriers as well, there really will be no cost difference for you. The Data plan you had before becomes you BIS plan (you do have to make sure that the carrier switches your data plan to the BlackBerry plan). The joy of BlackBerry is the data compression the device has. You use a lot less data for the same experience than an iPhone. So if you need to purchase a data amount, you can get by with less and still surf just as much.
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