Previous poster is correct, using Wi-Fi on your phone will not use data, remember that. You should have the My Verizon app. If you have limited data or texts it will be invaluable to keeping up with your usage. Welcome to the Droid family.
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Another neat app is 3G Watchdog. It monitors your 3G data usage and breaks it down for you. It can also be set to alert you when you get close to hitting your limit and its available as a widget too.