Here's the news from his website:
11th December 2007
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, 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 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 Iwould only entertain offers from very high-end experts in brain chemistry.
If anyone's looking for any charitable giving to do this year, may I suggest donating to a reputable firm that is researching this horrible, horrible disease.
Terry Pratchett's work holds an important part in my life - for many reasons I won't go in to.
Let's all send positive vibes and good thoughts to him.