The word coming from King himself is that after he publishes the final volume of the Dark Tower series (the 7th book, sometime in late 2004 I believe), he will officially retire from writing.
I don't believe him.
Anyone who's ever taken up writing as a hobby, either poetry or prose, knows that it's something that's in your blood & soul, and not exactly an easy thing to give up. My prediction is that he'll take a few years off, but in the end we'll see more from King before his time is up.