This is a discussion on Slot on side of phone within the iPhone 4S Forum forums, part of the iPhone 4 Forum category; Does anyone know what the slot on the right side of their new iPhone 4s? Looks like it could be an SD card slot....
Sorry about that, I thought 4s was GSM ready? How does this slot opened?
Use a small straight pin (sturdy enough to fit the hole) and press in. The door will push out as you push the pin in.
You will find your sim card laying inside the door. Careful though, as the door feels rather fragile. Remove pin and gently press the door closed to re-install.