As more and more evidence comes to light and more of Armstrong's former team-mates fall, it does indeed cast a considerable shadow over his career. However, as has already been pointed out, he is probably the most tested athlete on the planet and would have taken far more tests than his team mates. If they've tested positive, why hasn't he? The untestable and unprecedented factor that can never be measured is the physiological and psychological effect that surviving cancer had on him. He was an extremely talented cyclist before the cancer and the profound effect that the illness had on his physical parameters, ability to deal with pain and his drive could account for his performance. It's a sequence of events and conditions that have never happened to, will probably never happen again and can't be artificially replicated. Were his team mates doping just to keep up with him? It's possible.
I'm not an Armstrong fan but do think he could be an exceptional case or even a freak.