I once drew a diagram of the default Movable Type templates so I could make sense of all the nested divs and whatnot. (The templates are more complex than they need to be for modern browsers, but all the redundancy ensures Netscape 4 compatibility.) Of course I promptly lost the sketch before I could do anything useful with it.
Mediatinker’s excellent template illustrations go way beyond my little scrawl, and even slip a few CSS tips in there (class vs. id, for instance). This is fabulous stuff for anyone who wants to customize MT but is daunted by the layout.