and on a slash yahoo list, I figured I’d throw in my own two cents.
I first came across the term PWP in about 1984 or 1985. I was attending an early slash convention, IDICon. As soon as I arrived at the hotel I was informed I'd been drafted to be on the "Plot? What Plot?" (aka PWP) panel. (The panel was quite a lot of fun, too!)
Other definitions have cropped up within the last few years but unless someone comes up with a definition that predates 1984, "Plot? What Plot?" is the original definition of PWP.