Apple is now offering unlocked versions of the iPhone 5. They’re available at http://macte.ch/wmsJ0 .
The unlocked iPhone only works on supported GSM networks, such as AT&T in the U.S. When you travel internationally, you can also use a micro-SIM card for iPhone 4 and iPhone 4S or a nano-SIM card for iPhone 5 from a local GSM carrier. The unlocked iPhone won’t work with CDMA carriers such as Verizon Wireless or Sprint.
The unlocked iPhone 5 costs US$649 for 16GB model, $749 for the 32GB version and $849 for the 64GB model. The limit is two per customer.