Almost overnight, a booming market for unlocked iPhones has mushroomed in the United States and Europe. This week, hackers released the first software for unlocking the iPhone, freeing up the device from AT&T's network and enabling it to be used on any GSM network in the world. In the United States, sellers on Craigslist are hawking unlocked iPhones for between $450 and $700, a significant markup on the $400 devices.