The micro-SIM card is found under here, too, but its position next to the 1, 800mAh battery pack necessarily means you won't be swapping that out without shutting things down.
On the top edge are waterproof covers for both the micro-USB port and micro-SIM card, while the headphone socket, located between the two, is left open to the elements.
This means that if you purchase a local micro-SIM card (the ones compatible with the new iPad) you can make and receive phone calls on Skype or other VoIP service for about two cents a minute, in contrast to the high cost of roaming voice calls based upon all of the U.S. carriers pricing plans.