Why would you need a USB-powered soldering iron? To be honest, you could ask the same question about a lot of USB devices these days.
Still, if you were so inclined you could build one yourself with a simple battery-powered soldering iron, a USB cable and the instructions provided by GetLoFi. On the other hand, if you are willing to send an undisclosed amount of money to a shady Taiwanese exporter, there is at least one commercial version floating around out there. [GetLoFi and Megatone via Make]