From Today You can Buy Apple iPhone 4 Unlocked in US at $649. Both Black and White versions are available on the store, with the 16GB and 32GB versions selling for $649 and $749.
The main advantage of these unlocked iPhone 3 is for international travel, as the device is not dependent on any particular carrier. You can use it with any carrier.
You can find out more details about the unlocked iPhone 4 over at Apple.