I was gutted when I read that this morning. Been a Pratchett fan for more years than I can remember.
Hopefully they'll have caught it early enough that the drugs can hold it off for a while - my granddad has had it for about 7 or 8 years and the medication really seems to have noticably slowed down it's progress. God knows he'd have been like by now without it.
And of course Terry will have access to private treatment as well, so hopefully we'll be seeing his hat in bookstores for a while yet.
Read about this earlier today. Shame really, but it's not as if the amount of literature Mr. Pratchett has churned out over the years could easily be described as lacklustre. For me, his most memorable Discworld novel has to be 'The Reaperman'. Perhaps this will be one of his works that will... 'resurface', once the dementia has terminally set in.