This summer Z-Man Games will be publishing the board game Guards! Guards!, designed by Leonard Boyd & David Brashaw (aka Backspindle Games). Details about the theme and game-play are available at source.
But wait. Treefrog Games and Martin Wallace will be publishing Ankh-Morpork at Essen in October. And details about the theme and game-play are available at that source, too.
In the meantime, you can keep playing Thud, designed by Trevor Turan and released by The Cunning Artificer and then Mongoose Publishing. But if you’re really into Discworld, check out the Wikipedia entry on fictional games made up by Pratchett that were described in the series.