As predicted, the unlocked version of the iPhone 4 is now available and selling in the United States. Customers can purchase one of these and use it with any carrier they prefer. They can also use the iPhone internationally, on local carriers, if micro-SIM cards are available.
This is not a first for Apple. Unlocked iPhones have always been sold in other 85 other countries including Singapore, Malaysia, Hong Kong, South Africa, Belgium and Italy.
The Apple Online Store has a FAQ and details about the iPhones.
However, the unlocked iPhones are really expensive when compared to those available on Verizon or AT&T. This is because "carriers typically subsidize the purchase price of the phone when purchased with a service contract".
Unlocked iPhone 4s are priced at $649 and $749 for the 16 and 32 GB models respectively in the US. Right now, T-Mobile 3G service is not compatible with the unlocked GSM iPhone.
PC Magazine has a list of possible reasons why Apple might have started selling unlocked iPhones in the US, if you're interested.