This article from This is London quotes Robbie Coltrane discussing the future of Hagrid in the books:
(Note: Possible spoilers - highlight below text to read)
Time could be running out for Harry Potter giant Hagrid, it emerged yesterday. Robbie Coltrane, the actor who plays him in the films, said the character could be killed off in the fifth book of the popular series.
'I know about what happens to Hagrid, which is something author JK Rowling told me from the beginning to help me prepare me for the part,' he said. 'There is a death in the fifth book of a central character. But I can't say more than that.'
A true clue, or is Robbie Coltrane a red-herring master like JKR? Is he saying "I shouldn't said that" right now? :) You decide! (I already have - let's mark this with a big fat rumor, thanks.) Thanks to Alison for sending this in.