The first mention of the Emperor being Senator Palpatine are in the novelization of the first movie (first released, A New Hope, not Ep I). I used to have an old copy of that from the 70's (probably still have it somewhere) and I know that's where I first encountered the name Palpatine.
So, there's Palpatine who's established definatively as the Emperor in the novel (not to mention virtually every book or comic also referring to him as Emperor Palpatine), and there's Darth Sidious, who even discounting the fact that he's played by the same guy, looks and acts like the Emperor! Sidious=Emperor=Palpatine. What's the big f'n problem some people have with this?