South African Oscar Pistorius has qualified to compete in the 4x400m relay at the London Olympics. Modern technology is amazing. There is no way in looking at him walking running that would tell you that he had prosthetics for both legs below the knee, unless you look closely.
Here is Pistorius running at the 2001 World Championships.
Update: This article explains why Pistorius’s prosthetic limbs were ruled to be eligible and how each new development will have to be decided on a case by case basis.