If you check the AT&T section of Howard Forums, you'll find that AT&T seems to know the IMEI of Nexus One phones and will automatically send you a message stating that you require the $30 smartphone plan. You don't even need to register the phone. AT&T knows.
As for not using data: All Android phones use data the moment you turn them on because of the requirements of apps. The only way to avoid this are (1) NOT to set the APN address of the carrier (AT&T), 2 use an app called APNdroid (that can turn on and off the APN), or 3 tell AT&T to block data (I am not sure if that is allowed anymore). But if you do that, why even buy a smartphone.
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