It looks about the size of a Firewire port.
My guess is something in the form of a memory port, since this thing might be going on the internet they left some room for someplace to store data besides the "cart".
The only other thing comes to mind, and the likely reason it right next to the headset port is that it is for a full headset, with mic for louder areas so you voice doesn't get drowned out reducing the need to yell.