From Lilja's Library:
Scribner has announced that 11/22/63 will be out in paperback in October 2012 and also that both The Talisman and Black House will be re-released in hardcover in November. That and the fact that there are no other King book scheduled for this fall might get the optimist hoping that a third book about Jack Sawyer is closer than we know… Well OK that is probably not the case but you can hope, right?
From the Stephen King message board February 9, 2012.
Stephen and Peter haven't been able to coordinate schedules to begin working on the third book as they've each had other projects that needed their attention. I don't know if they have discussed plot development yet but even if they had, it would probably be too early to release any info.
I doubt we'll see the next Jack Sawyer book any time soon. They are Dark Tower books and I'd predict a bit of a wait after Keyhole comes out for another Tower book.