I think that only 4 people will know by the end of Ep 3 that Palpatine is actually Darth Sideous ....Vader, Yoda, Obi-Wan and Palpatine himself of course. Palpatine will be forced to reveal himself as Darth Sideous to Obi-Wan inorder to rescue Anakin. There is a lot of evidence to support that Darth Sideous continues to disguise himself as Palpatine right up to the moment he arrives on Death Star 2. 1.Leia tells Darth Vader that the Emperor will not be impressed when he finds out that Vader has attacked a ship on a Diplomatic Mission. I doubt she would be so assured if she knew the Emperor was actually a Dark Lord of the Sith. 2. The senate although powerless is still in existence at the start of Ep 4. What would be the point of keeping up the pretense of a democracy if everyone knew you were a Dark Lord. 3. Yoda warns Luke not to underestimate the powers of the Emperor or suffer his father's fate he will. Yoda knows who Palpatine really is but obviously the Emperor is considered to be an ordinary man .. a politician...it is not widely known that he is in fact a Dark Lord. 4. The General who pokes fun at the force in front of Vader on board Death Star 1 surely would not have done so if he knew his Commander in Chief was also a Dark Lord of the Sith.