You guys, Natasha Richardson is already dead, and at a young age too. Now since we don't really know what kind of person she was, I think we need to give her the benefit of the doubt. It's all in G-d's hands now anyway. That goes for all us when we eventually pass.
Besides which, if we keep saying "yimach shemo" to everyone, then it loses its meaning.
In terms of wishing cancer on people who we do not know, I'm inclined to agree with Carreg. Remember, we're the good guys here. Let's save our curses and our ill wishes for people who we know, beyond a shadow of a doubt, deserve it.
Now since you've all gotten to chime in on this matter, I think I'd better lock this thread.