Rowling has said on her website that another character will not survive “Half-Blood Prince”.
If its anything like “Order of the Phoenix” it won’t be any of the big three. (Harry of course lasts through the series and I think Ron and Hermione are safe until then too.) Nor do I see Dumbledore or “He-who-should-not-be-named” biting the dust 'til the end either. I do think it will be someone semi-major.
My guess is Neville. Perhaps he will somehow try and seek revenge for his parents fate. Or the Death Eaters could come after him for similar reasons.
Another friend of mine thinks it will be Draco because his usefullness as a foil for Harry has passed.
What do you think?