The Wind Through the Keyhole will be the next DT book and from the SK site we've got this from the moderator.
"Stephen has given me permission to pass along that he has an idea for a new Dark Tower book, the working title of which will be THE WIND THROUGH THE KEYHOLE. He has not yet started this book and anticipates that it will be a minimum of eight months before he is able to begin writing it."
I think it's waay to early to think much about a book that isn't written but I would expect the following to happen:
#1) Release date late 2011 or later
#2) Limited ed by DMG
#3) Hang onto your numbered Little Sisters to gaurantee a copy of this one at publisher's price. (somewhere between $250 - $350)
#4) A title change before it's all said and done.
Fearless predictions from your fearless leader (Put's his crystal ball away)