Did this by accident, but it is sweet. Put my microsim in an adapter and put into a friend's iPhone 2g that had to be activated through AT&T. Actiavted the phone and put it back in my iPhone 4. Next day I call AT&T and on the ip4 I am still part of the family plan for minutes and text but now the phone is on unlimited data and 200 text messages for only $20. If I get any line activation fees on my bill I will play stupid and have AT&T refund them.