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Have you downloaded any media? apps aren't that big, but movies and a lot of music will eat at data.
First, make sure you are actually connecting to your wireless network by going to menu>settings>wireless & networks>mobile networks and un-check the box that says data enabled. This will shut of data over AT&T's network. Make sure your WiFi is enabled by going to menu>settings>Wireless & networks>Wifi settings and make sure your home network is listed and it says connected. Now try browsing the web and downloading an app. If either of them fail then you aren't getting data through Wifi and it could account for the heavy data use.