I believe it'll either be Hagrid or Mrs. Weasley who dies in the 5th book . . . not that I'm looking forward to ANY death, particularly one of a character we've gotten to know.
I've read that the person who dies is a "big fan of Harry's" and that the death was "hard to write", according to J.K. Rowling. Hagrid, as has been stated, is J.K.'s favorite character, which would be hard to write. Likewise, J.K. lost her mother at a young age, so I imagine writing the death of a mother would be very hard for her as well.
I've just skimmed this thread and I know Hagrid was mentioned, but I'm not sure if Mrs. Weasley has. She's always been very motherly toward Harry, so I think it would hit him (and all the Weasleys) very hard if she were to die. It might be what prompts Ron to take a real stand against Voldemort (not that he wouldn't already, but you know what I mean).
Those are my 'top two', but if we're discussing 'big fans of Harry's' there is also Lupin and Sirius to consider . . . they are both big fans of Harry's as well. Additionally, like the people mentioned above, their deaths would certainly be hard to READ, if not to write! I'm dreading finding out who it is at all!

It'll be the last time we read about a certain character (well, as a person as opposed to a ghost) . . .
