Not sure what new, if anything, there will be, but Scribner is publishing a new 10th anniversary edition of On Writing on July 13, 2010. What's interesting is that it's a hardcover, not a paperback, as it's usually done with such reprints.
There is only this short description:
http://www.amazon.com/dp/1439193630/A Scribner Classics edition of Stephen King's critically lauded, million-copy bestseller about the craft of writing.