Prosthetic devices have reached the point where they don't just help people in need. They can make them as fit -- or fitter -- than everyone else. South African sprinter Oscar Pistorius, an amputee with question-mark-shaped carbon fiber prosthetics below his knees, may qualify for this summer's Olympic Games. Is that fair to competing runners? Neural implants are helping to cure mental deficiencies. Will they also be used to enhance the functions of "normal" brains? How long will it be before human beings can decide to transcend biology? What are the practical obstacles and the ethical issues?