Yes, it is, and I have personally met athletes who would practice this sort of doping. People really understimate the boost you get by training on a high altitude. I have experienced this myself, and it was amazing to say the least. I was living in an altitude of 3000 m above sea level (about 10 000 ft), and I was training there a lot. When I came back down to sea level, I just did not get tired - I would run all the time, and do stuff as if I just could not get tired! Unfortunately, this lasted only a couple of months, and then I became a normal guy again. So to answer your question, yes, it works, and yes - athletes do it all the time.