From Neil Gaiman's blog:
Several people wrote to ask what I thought about Eoin Colfer writing a new Hitchhiker's book--for example,
http://www.timesonline.co.uk/tol/arts_and_entertainment/books/fiction/article4773155.ece In regard to the above, did they ask you to do it, and would you have accepted if they had?
Nobody asked me to do it, but then, when Douglas asked me if I'd like to adapt Life, The Universe and Everything for radio I said no, and that was with Douglas alive and asking. (Dirk Maggs did it, and did an excellent job.) It seemed a thankless task.
I like Eoin very much, and wish him well with the book. He'll probably write a sixth Hitchhiker's book with more enthusiasm, and certainly faster, than Douglas would have done. But it won't be a Douglas Adams Hitchhiker's book.
For the record, if I don't get around to writing a sequel to something while I'm alive, I'd very much rather that nobody else does it once I'm dead. It should exist in your head or in Lucien's library, or in fanfic. But that's me, and not every author feels the same way.
Not every author feels that way, indeed! There's nothing like an author I already like making me squee by writing positively about fanfic. ;)