According to the Guardian, PTerry has been forced to pull out of the forthcoming Discworld convention due to his Alzheimer's.
I remember hearing he had been diagnosed - having been my favourite author for almost my entire life (with only Frank Herbert, who I had read before discovering Pratchett, running him close) - I was absolutely devastated. It's a horrible thing to see your heroes brought low by something so horrible. For a man like Terry, so clever and quick-witted, to be diagnosed with this condition felt like a horrible, cruel twist of fate.
He'll always be my favourite now. I can't imagine any author ever coming close to the love and affection I feel for the Discworld (and other) novels he's produced. I must've re-read them all more then 20/30 times each, easily, and I can't imagine a time in my life where I'll never want to read them again.
To say I'm gutted to hear it's taking further hold of him is an understatement. I has a sad.