Okay, this was a given, but I saw proof last night in a local bookstore. While thumbing through one of the RPG sourcebooks (I think it was The Dark Side Handbook, or something like that), I happened upon the following quote (which is from memory and, therefore, probably way off):
"Years before he had the Emperor's Hands [eg Mara Jade, roughly the time period of the Classic Trilogy], Palpatine used Darth Maul as his instrument."
Wow! As if "I will make it legal" wasn't enough evidence. I'm kind of surprised LucasFilm allowed this--they've been keeping a tight lid on the supposed mystery of Darth Sidious up until now.