Here's the post Mr. Pratchett put up on his artist's website, http://www.paulkidby.com/news/index.html :
11th December 2007
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 earlyonset 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, Iexpect 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 shouldbe interpreted as 'I am not dead'. I will, of course, be dead at some future point, aswill 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.
Now I know that this is going to be all over the web; in fact, I've found it in quite a number of spots. But I'm such a huge fan that I couldn't go without sending him my best wishes, even if he doesn't know me from Adam.
Good luck, Mr. Pratchett. I look forward to your next book.