It's interesting that a few people are comparing wham warnings to the blurb on book covers, because I never thought of them like that before. I can see where you're coming from; there are certainly parallels. However, I've always thought of a wham warning as a notice that bad or violent stuff is going to happen in this story, and that it will likely happen to one of the main characters. Book covers, on the other hand - and I've just checked a few at random from my bookshelves - appear to set the scene for the reader and pose a few unanswered questions to which you can only find the answers by reading the book. They're there as a teaser; as a sales tool, really. They don't, so far as I've seen, tell you specifically that a rape, murder, character death etc is going to happen.
I think the book cover parallel works best with writers' introductions; most of us give them, although we don't necessarily use them to set the scene for the reader. Often they're used to inject a personal note - why we wrote the story, for example, or to thank the people who assisted in the writing of the story.
Perhaps it would be nice to start a tradition of book cover style intros as well as wham warnings. Nothing obligatory, of course, although it's just occurred to me that the archive requires some sort of intro, so it's something most writers will have to do at some point anyway. What do people think?
Yvonne
