This is very, very sad news; but, as Talia says, I am holding out that his creativity and writing help him stave off the disease. There's a lot of progress being made in holding Alz. at bay, esp. if you catch it early; so, I think he's got many years and many books left in him.
My first book was "The Color of Magic," and I was pretty immediately in love with the world. I've read almost all of his work now, with the exception of a scant few I just haven't been able to get my hands on, and he's one of my all-time favorite authors. He's certainly my favorite humorist and my favorite political commentator in the SF genre, and he manages to get those two things to be complimentary instead of forced. That's talent.