Apple has begun selling unlocked iPhone 5's on their web site (and presumably in their stores, as 9TO5Mac is reporting).
The phone comes in black and white, 16GB, 32GB or 64GB, but GSM-only. From Apple:
Because the phone is GSM-only, in the US it will work on AT&T and T-Mobile, but not CDMA networks like those run by Verizon and Sprint.
About the Unlocked iPhone
The unlocked iPhone includes all the features of iPhone but without a wireless contract commitment. You can activate and use iPhone on the supported GSM wireless network of your choice, such as AT&T in the United States. The unlocked iPhone 4, iPhone 4S and iPhone 5 will not work with CDMA-based carriers such as Verizon Wireless or Sprint.
If you don't want a multiyear service contract, or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone is the best choice. It does not come with a micro-SIM card for iPhone 4 and iPhone 4S, or a nano-SIM card for iPhone 5, so you'll need to get one from any supported GSM carrier worldwide. To start using it, simply insert the card into the slot on your iPhone and turn it on by pressing and holding the On/Off button for a few seconds. Then follow the onscreen instructions to set up your iPhone.
According to Engadget, the unlocked iPhone 5 will work on some LTE networks in some countries, but which is not clear.