So I got my new iPhone 4s today and am giving my iPhone 4 to the wife. I put the SIM card from my iPhone 4 into the 4s and it worked great. I put the new SIM card that shipped with the 4s in my old iPhone 4 and then called AT&T to have the number switched from my wife's old phone to the iPhone 4 (since her old phone had the larger SIM, not the micro SIM). Seems like everything is working but while I am on the phone with the guy from AT&T he recommended that the SIM card that shipped with the 4s should stay with the 4s (something about better network speed and connection). He also said that since both SIMS are now active that I should go to the AT&T and get two new SIM cards and call back with the SIM numbers and IMEI numbers. He seemed to believe that there would be issues performance wise (data speed) if the two cards were swapped and that I should start fresh. Now, I've never heard such a thing. I always thought the SIM just put you on the network and if you were getting/receiving calls and data you were good to go. I'm tempted to call shenanigans on this and not mess around with the whole get two new SIM cards advice. Anybody every hear anything like this or believe the AT&T rep is on to something? Would you go get in line for new SIM cards or roll with things the way they are since the phones seem to be working OK?