No one has ever been caught in the top level but there's enough 'unexplained' events that do make you wonder if the bike is all it looks to be!
Personally I wouldn't think any of the top riders do it - they may have done previously but I think they know the authorities are looking out for it now, but you never know for sure do you!
I remember Hill and Bedford and they didn't cheat and I thought Viren was amazing in his first Olympics - we all did - but doing nothing for four years then winning both events again at the next Olympics...something fishy going on???
I don't think he ever admitted to it or was caught but it's clear he cheated.
Maybe if he was around these days he'd try to get a motor in his running shoes instead of spikes perhaps!