Fasectious (sp?) answer after everyone's experiences (sorry)...
My DS is a bit obsessed with death at the moment, he'll talk about dead creatures he's seen, and does a lot of thinking about what has happened to the baddies pushed down a hole in Star Wars!
Because I am total athesist I tend to fiocus on the science bit, ie the difference between being dead and alive. (like how does he know the bird is dead, is it moving, breathing, can it see, etc etc). Now he is grasping the fact that when something is dead it ceases to function, and can't 'do anything'.
I agree that it's like sex ed, that you have to 'drip feed' it. I have no intention of telling him about any kind of afterlife, because I don't believe that that is true.
I don't think that being dead is anything to be afraid of, and I hope to convey that to my son. Obviously being alive is better, you can do lots of things, but being dead just means that nothing else will happen.
Regarding remembering others, I agree with bosomsbythesea that it is the memories that others keep that keeps your 'soul' alive.