Going Postal will be out soon, and starts in Ankh-Morpork with a character named Moist von Lipwig.
Wintersmith is in the works, and will be another Tiffany Aching story.
There will possibly be a Vimes murder mystery, possibly named Thud .
There will almost certainly be another Tiffany Aching story after that, and maybe another one after that (depending on how much story fits into a book).
(and eventually they'll all be out in paperback and we can buy them all !)
(Edited: to fix a mis-remembered book name !)