I had the same thought as D. D. Don't open up a discussion of the hypotheticals as if your older one is going to get a vote on this. If they trash the idea, then what? And even if they like the idea, when the baby comes and is annoying as heck, then what? You can't say to the older child, "Well you were in favor of this so you don't get to change your mind."
It's an adult decision, and it needs to stay that way. I also don't think how a child takes the news is any indication of anything. They have no frame of reference, they can't imagine what it's going to be like. There should be no expectation that they will be excited at the news, that they should be able to fantasize about what life will be like, or that it will be all fun or all misery. It just IS. How they feel at the IDEA is one thing, how they feel when Mom and baby get all the attention is another, how it impacts their life in the short run is yet another, and how it affects them in the long run is still another.
The big things to consider when announcing to and preparing an older child is to think about all the things that other people will say as well as you! Others will expect him to be thrilled, or miserable, some will say "Your life is never going to be the same", some will say "You'll be a terrific big brother" (which is meant as a compliment but tells that kid that he's got a job now that he didn't sign up for), some will say "You'd better start learning to deal with poopy diapers" or stuff like that - so all of these things affect the child. It turns into an information overload.
I wouldn't lay too much groundwork other than discussions in general of what are parent decisions (jobs, finances, medical care) and what things kids get input on (some input o meal choices, types of vacations, movie choices just to name a few), either from the get-go or as they get older. Those are helpful family dynamics topics anyway, and it helps older kids navigate the maturation process from being powerless to being totally independent as an adult, and all of the development in between.