Those serial numbers, MC603, MC604, MC605, and MC606, correspond to the international GSM versions of the 16GB and 32GB models in both black and white. But they are missing the 'B' or 'C' characters that denote a British or Canadian device. So we can infer that these are unlocked iPhones for Apple's home market.
But why would Apple do this after avoiding an unlocked U.S. device for so long? One theory is that it is partially at AT&T's behest. This would enable AT&T iPhone customers to use international service providers while abroad, and get more competitive rates than AT&T can offer to travelers.
Also, if the T-Mobile deal goes through (and we imagine it will), those T-Mobile customers will be brought into the big blue fold anyway. So AT&T can soften their authoritarian image, while building some good will with customers, Apple, and the FCC at the same time.