I don't think that he doped at all. His cancer obviously had a positive effect on his physique and mental attitude.
Also, you can look at Tiger Woods with his different attitude towards training for golf as a reason for his massive success - there is no reason why it cannot be the same for Armstrong with his attitude towards training and the team which was built up around him (I would use this argument to defend his first couple of wins, but once other teams caught up he must have had to pull something new out of the hat).
The same can be said for England's 2003 Rugby WC success - they employed sports science techniques and sports psychologists when the other teams didn't; having the mental edge is often touted as the difference between winning and losing.
Recently I have been thinking about something - Armstrong and Contador have both had brain surgery, and I wonder if that may have had any influence on other things. (Don't attack me on this, because it is just a question and I doubt any amount of current medical expertise could ratify or reject it.)