Well, according to an interview with JK Rowling that's on MSNBC, Deathly Hallows may not quite be the last book.
She is strongly considering getting her various notes together to create a Potterverse Encyclopedia to cover all the elements that she had to excise from her books, citing as an example the character of Dean Thomas, whose background was far more complex that what we saw in the books, as well as more of what happens to various characters after the main events in Book 7.
