There have been people caught using performance enhancing drugs. I think there's been 3-4 people that have either been stripped of their medals or been ejected from the games.
The problem is that the testing methods and techniques are always playing catch up.
There are ALOT of athletes that have been cheating, but have figured out how to avoid a positive drug test.
The best example of this was Marion Jones, the american track & field athlete. She won 4 world championship medals between 1997-8 and 5 olympic medals in 2000 in Sydney, and was tested hundreds of times without getting a single positive result. But last year, she confessed to "doping" during the 2000 olympics in Sydney and was stripped of her 5 olympic medals.