Yep. You can do that. Just tell them to have it delivered to you instead of to the store. That way, you can activate it at home, whenever you want to, instead of immediately then and there.I'm upgrade eligible for the 4S but I was wondering if I could buy a 4S but not activate the 4S until later, keeping my 3GS active for a while (month or so). Does anyone know if ATT will let you do that?
After it is activated you could always go to the AT&T store and get a new regular SIM for your line, or you could use a micro-SIM to regular SIM adapter and use your 4S SIM in your 3GS.If the activation has not been completed within  days after the shipment date, we will complete the activation process for you. Your monthly recurring charges and applicable service commitment will begin once your line is activated.