Obviously, I've not seen the film yet, but the novelisation of Revenge Of The Sith makes quite clear something I'd suspected since I saw The Phantom Menace.
Darth Sidious is Chancellor/Emperor Palpatine.
No, that's not the funny bit. The funny bit is that this is supposed to be a big old M Night Shyamalan surprise.
Oh, George... and the trailer had me convinced that you weren't going to cock this one up...