Upon insertion of your SIM card and powering up, your phone reports its IMEI number to the network. These days the system automatically adds the correct data plan shortly after putting your SIM in a phone that requires it (which is any AT&T branded smartphone.) For the great majority of people, anyways. There's only a very few of us who somehow appear to be immune to that. If you buy an unbranded Galaxy S you can keep the $15 medianet. The only unbranded phone that does trigger the automatic plan change is the nexus one. All others are still good to go.