One of my old uni tutors had an interestin view on this sort of thing:
What is the definition of a performance enhancing substance? In simple terms, it's a substance that enhances performance! So - is water a performance enhancing substance? Lucozade? Where do you draw the line to say that this is "natural" and this isn't? If you deprived cyclists of water, they'd perform worse.
I personally feel that everything should be allowed and that way there's a level playing field. Sportsmen are adults. They are well aware of the dangers of performance enhancing substances and if they want to take them and risk their long term health for the sport that is up to them. (in my opinion, it's not any different to rugby and football players risking injury on the field, racing drivers risking accidents etc.) As far as I'm concerned, that would be more entertaining for the spectators than the witch hunts that happen years after an event.
As an interesting side note, there is a gentleman (whose name eludes me at the minute) who was adviser to a lot of top class bodybuilders. As I'm sure you're aware, bodybuilding allows the use of many IOC banned substances. The nickname of this particular gentleman is "The Chemist". He makes no claims to be a personal trainer, nutritionist or anything else. His current client is Usain Bolt. Make of that what you will.