Hmm . . . well, I've been researching nursury rhyme origins (thanks to Holly and her obsession with having them in horror stories) and I've found out a lot so far! Perhaps I'm just bit behind everyone, buuuut:
Jack and Jill is actually a story of the French Revolution. Jack (King Louis Auguste) and Jill (Marie Antoinette) didn't fall down a hill, they were beheaded. A happy ending was given to the poem (something about Jack going to bed and bandaging his head) so it would seem more appropriate for children to be singing.
Enjoy the morbidity as much as I do. LOL