The 4S has a small hole in the top edge next to the headphone jack. The hole is about the same size and position as the SIM-eject hole on the 3GS, but on the 4S the SIM tray is in the side, so it's not that.
Anyone know what it's for? Was it on the ip4 too?
Anyone know what it's for? Was it on the ip4 too?