Just read this article on BBC, and what really caught my attention is less Sir Pratchett's views on assisted suicide but, rather, that he quite pointedly brings himself and his own condition as an example of someone who should be allowed to end his life for medical reasons. He's not really concealing the fact that he's seriously considering assisted suicide when the Alzheimer's gets really bad.
Oh tut tut, Roman, Mal posted this on the site back in July! Terry Pratchett Books - Terry Pratchett Views on Assisted Death
He did a recent interview on the subject which you might find interesting. BBC NEWS | Programmes | Daily Politics | Pratchett on how he wants to die