Apple says that if you don't want to wear the handcuffs associated with a two-year service contract, or you want to use a local carrier when out of the country, that you get the appropriate SIM card (nano for iPhone 5) and install it on the unlocked variant of the phone. The SIM card does not come with the unit.
Apple is limiting these online orders to two per person. If the thought of being tied up with a two-year contract makes you sweat and you plan on using one of the supported GSM networks, you can order an unlocked version of the Apple iPhone 5 by clicking on the sourcelink.
source: Apple via AppleInsider