In the US, Apple has begun selling an unlocked GSM version of the iPhone 4. The cost is $649 and for that you get the unlocked iPhone 4 compatible with GSM networks. It comes with no microSIM card.
Although the GSM version wont work on all networks in the US, the benefit of this version is that when you travel abroad you can put in a microSIM from another country inside and it will connect up to those networks.
The 16GB model is $649 while the larger 32GB model costs $749 and both models are available in 2 - 3 days. The white version is priced the same but takes up to 5 days to ship.
The iPhone 4 is available now over at the Apple store.