Apple on Tuesday began selling unlocked iPhone 4 handsets in the U.S. a day earlier than expected. The smartphone is now available in 16GB for $649 or 32GB for $749, and both models come in black or white. Other manufacturers, most notably Nokia and Sony, have sold unlocked smartphones in the U.S. for quite some time. Neither company has seen much success with unlocked handsets thus far, though Apple’s approach is somewhat different. Apple’s primary sales channels in the U.S. are undoubtedly carriers, and the availability of unlocked smartphones simply addresses the needs of a small secondary market that is willing to pay for phones outright rather than spreading the cost of their devices over two years. As an added benefit, buyers of unlocked iPhones will be able to switch freely between GSM carriers, though some surgery may be involved with carriers that do not offer micro-SIM cards. With just a few months between the start of the summer and the expected launch of Apple’s fifth-generation iPhone, the availability of unlocked devices will allow also users to purchase iPhones now and then make a subsidized purchase in September, or whenever Apple finally does release the iPhone 5.