If you like digging through design material for classic games, Ultima Aiera has a bunch of original desgn and technical docs from Ultima 6, provided by Origin programmer Bill Randolph.

I must admit to being totally ignorant about this but Mark writes in to say Ultima VI Online is, well, back online.  As I understand it, this is an MMORPG built using assets from Ultima VI (feel free to fill in details in the comments).  Details can be found at Ultima: Aiera, an excellent Ultima projects news site.