http://www.electronista.com/articles/08/01/23/macbook.air.usb.details/

The MacBook Air’s USB port has been modified to deliver more than 5v so it can power the SuperDrive accessory without it needing another power cable. This is the main reason you can’t use the Superdrive on any other computer. Annoying tradeoff, but it shows how far Apple will go to help with clutter.