As a Cyclist and Cycling official I am well aware that Cycling tests more than any other sport, which is why there have been so many caught since the bad old days in the 60s. Contador for example lost the Tour de France 18 months ago after a prolonged wrangle over Clenbuterol (which he claims was ingested from contaminated beef he ate in a restaurant) He tested once during the three weeks of that Tour de France, during which he would have been subject to tests on more than a dozen ocassions. The positive was for for a trace of the drug, which at that time was 20 times lower than the Union Cycliste International proscribed limit. World Anti Doping successfully eventually found him guilty in conjuction with the UCI . I was surprised to discover that Clenbuterol is commonly used by farmers in some countries to fatten their cattle for market.
Continental Pros in the 60s, 70s and before used to race 3 to 4 hour races every day during the summer to make a living, often driving huge distances round Europe between races and took allsorts of conconctions to counteract the lack of sleep and boost performance. Thankfully those days are now history.
As far as Dwain Chambers and the others listed above go. They should all have been aware of the BOA stance regarding future exclusion from selection as Elite athletes, and in my opinion exclude themselves from the Olympics sine die.
All Professional or Elite Sportspersons are responsible for what they ingest, and should be big enough to take responsibiltiy when caught. I feel that bans should be life time for first offences.
The list of Proscribed Substances is extensive and includes all the masking agents the sporting authorities are aware of.