JK Rowling has recently stated that she has written a new novel that will be her first book aimed at an adult market. She seems to have just about finished it, but has given no information about when it will be published, the title or what it is about. This shouldn't be a surprise though. If you remember when the Harry Potter books were coming out, Rowling and her publishers were really careful about how much advance information they gave out, and they seem to be doing the same in this case. You can sign up to her website though, if you want to get updates as they appear.
As some readers have pointed out, writing for adults isn't a huge departure from what she was doing before, because the Potter books got more mature in style as the characters grew older - and of course the readers along with them. So the first book is aimed at 10-year-olds and the last more at readers in their mid to late teens - and besides, millions of adults have enjoyed the Potter books too.