A reader sets me straight. Seems the unlocked iPhones are being redeployed in non-U.S. cellphone services. Pulled from comments:

You are quite uninformed. No amount of soldering or hacking is going to get an iPhone to work on Sprint or Verizon. The iPhone is a GSM phone. Sprint and Verizon are CDMA networks. It's not possible. The only other GSM network in the US is T-Mobile, and there is zero indication that people are leaving AT&T for T-Mobile in droves (quite the opposite, actually). The vast majority of people that are unlocking iPhones are doing so to use them in foreign markets where the iPhone is not available. Pure and simple. That people are going through so much trouble and expense to get their hands on Apple's product is a GOOD PROBLEM to have. Welcome to the 21st Century.