Look, we already know he's writing another Discworld book, and that it's *not* Raising Taxes. Tweets from a month or so ago confirm this. He hasn't said what it is yet. Possibly it'll be an end-of-2013 release, but I'm suspecting perhaps it'll be out in 2014 instead.
Unseen University Convivium 2012
University of Adelaide, South Australia, 6-8 July 2012