I inquired with my carrier about the nano sim, they said it is thinner than the current micro sim. You cannot use a trimmed down micro sim in the new iPhone as it will be too thick to fit in the tray/slot. I'm not saying that this is true, but this is what that carrier told me.
My carrier will replace my micro sim with a nano sim for free.
Sent from my iPhone 4S or iPad 3rd gen, whichever I happened to have in my hands at the time, using iPF.net