The Sole is essentially what the name implies: a special shoe that is designed specifically easily slip onto the blade and provide the athlete with comfort and enhanced performance just like their running shoes. It may look like Nike just took the bottom half of one of their running shoes (i.e. the sole) and attached a fancy set of clips so that it attaches to the blade; however, if you watch the clip above, you can see that a lot of thought went into the design of the Sole to make it user-friendly, comfortable, and improve performance.
It is available now. Check your local prothetists' office who carry Össur's Flex-Run blades.