It appears that Apple will be launching unlocked version of the iPhone 4 to Apple Retail stores sometime this week.



From MacRumors:

As it turns out, there was a misinterpretation with the parts numbers that Apple sent to retail stores. The part numbers actually correspond to unlocked iPhone 4s, not new MacBook Airs. Apple has sold unlocked iPhone 4s internationally but never in the U.S. Unlocked mobile phones are simply less in demand in the U.S. due to the fragmentation in service amongst U.S. carriers.

"Unlocked" simply means that a mobile phone is not tied to any particular carrier. In Europe, customers can buy an Unlocked phone and easily switch between carriers by simply replacing the SIM card. In the U.S., users could theoretically switch between AT&T and T-Mobile in this manner for voice calls, but 3G service is not compatible between the two carriers. This is what makes the sale of unlocked iPhone 4s in the U.S. so strange.

  Apple to Offer Unlocked U.S. iPhone 4s Starting at $649?