They tried to make an effort a few years back, banning diuretics in 1993 I think. There had been a few of the big names absolutely fuck themselves by having zero water on board. Try taking a few lasix pills and not drinking anything plus the whole sodium loading effort, and being able to function normally. It doesn't happen. That year the Olympia was pretty average, competitors were a lot smoother, not as ripped and dry as normal. Not much of a spectacle really.
As far as other drugs, testosterone, GH, thyroid hormones etc, no, I don't think the IFBB (professional governing body) has ever bothered. From a business perspective it would just ruin everything. Typing this I feel a bit guilty about watching these dudes who are taking years off their lives with the stupid amounts of shit they use!
The Olympia really is a circus. It's a huge marketing vehicle for Joe Weider and co, the winner is usually not the best man there. Still fun to watch though.
http://www.bodybuilding.com/fun/2009-olympia-top-10-predictions.htm
Not a great top ten preview, but gives you some idea of the competitors. My pick is Phil Heath to win.