Given that Neil Gaiman's only just finished the scripts, I suspect casting is still a ways off. That being said, Neil did say either on his twitter or in an interview that he's attending a readthrough of the script later this year - whether that's with an official cast or just temporary actors, who knows?
I suspect you *might* be able to get a little bit of information out of Rob Wilkins and Rod Brown at the UK Discworld convention in a few weeks' time, however...