In the words of the man himself, "An Embuggerance"

Terry Pratchett has left a note on paulkidby.com saying that the 'phantom stroke' that he suffered earlier this year was in fact a very rare form of Alzheimers that he has since been diagnosed with.
Blogger is refusing to let me add links today so I'm including the main body of the text below,

"Folks,

I would have liked to keep this one quiet for a little while, but because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news. I have been diagnosed with a very rare form of early onset Alzheimer's, which lay behind this year's phantom "stroke".
We are taking it fairly philosophically down here and possibly with a mild optimism. For now work is continuing on the completion of Nation and the basic notes are already being laid down for Unseen Academicals. All other things being equal, I expect to meet most current and, as far as possible, future commitments but will discuss things with the various organisers. Frankly, I would prefer it if people kept things cheerful, because I think there's time for at least a few more books yet :o)

PS I would just like to draw attention to everyone reading the above that this should be interpreted as 'I am not dead'. I will, of course, be dead at some future point, as will everybody else. For me, this maybe further off than you think - it's too soon to tell. I know it's a very human thing to say "Is there anything I can do", but in this case I would only entertain offers from very high-end experts in brain chemistry."

Alzheimers is a horrible thing to happen to anyone so fair play to Terry for keeping a little perspective on it at the moment. Good luck to both him and his family.