As I work in a mostly arid climate, although the past few springs have been very wet, I've never seen this happen to a body. In other parts of the country that are more humid and cool, saponification happens. Yes, a body can turn to "soap". Click the link for an interesting article. If you are into this sort of thing. It is also known as "grave wax" or most professionally adipocere.
Well not really soap that we wash with. Here is the Wikipedia article.