Update: Scott writes:
IIRC he was Senator for Naboo, that would suggest he was from Nabo originally.The Star Wars Databank concurs:
http://www.starwars.com/databank/character/palpatine/
>> Palpatine was the supreme ruler of the most powerful tyrannical
>> regime the galaxy had ever witnessed, yet his roots are
>> extremely humble, traced back to the peaceful world of Naboo.
(which also sadly puts to rest your theory that Lucas might decide that Sidious != Palpatine).
Actually it only proves that Sen. Palpatine == Emperor Palpatine, which I don't see any way around. But Darth Sidious could be someone completely different, who thought he was using Palpatine as a dupe... until Palpatine kicked his ass and took over! See how much more interesting that is?