Yesterday Apple began selling an unlocked version of the iPhone 4S. That means that you can use it with any carrier, and you don't need to sign a contract. But it will cost you: prices start at $649 for the 16GB phone. You can also buy an unlocked iPhone 4 for $549 and a 3GS for $375. However, the unlocked phones will only work with GSM carriers, such as AT&T and T-Mobile in the U.S.