In the comments, someone asked about how to make a Watteau pleat. Actually, it's not that difficult. I started out studying pictures like the one in my first post about it. Trying to determine how it was built up and how far it was stitched down.
However, on my recent trip to the book store (ah, books and fabric, my two great weaknesses...) I found this great book.
In it were pictures like this one, which shows how the back of an orginal sack back dress was folded. I did mine the same way, before I found the book.
Essentially the pleat is completely straight, all its parts are the same width for their entire length. I'll have to make some little drawings to show you how to engineer that thing into a dress, but its about time I go to work. So more about that tomorrow.