Decided on Carpe Jugulum in the end and OH took Guards, Guards.
When I was getting it signed I paraphrased and said that everyone at the MobileRead Forums said 'hi', and he was vaguely interested enough to ask what that was. I summarised, and also said we'd love it if all his works appeared as ebooks, and he said he thought 'they were working on it'.
The talk was really good too, he's reassurringly funny in real life and was mainly talking about the newly released Nation, which is non-Discworld. He even sang

He also told us about the Discworld in progress which is about