This not true. There is no "Magic" iPhone SIM. This rumor comes up with every iPhone since the orignial iPhone need to be paired with the SIM during iTunes activation.You CANNOT use a SIM out of your old phone. They will not work in the IPhone. The phone will come with a SIM installed and you just go through the ITunes activation process.
If you have an active AT&T SIM from the last 24-36 months it will work just fine. Only the really old "AT&T" Blue SIMs are not going to work. So the Cingular Fireball and the 3G logo AT&T SIMs all work just fine.
Dave