I do spend a fair amount of time letting the person know why I ask the question, where it comes from (Reid Hoffman's book), and why I'm asking. I also position the question as being about a time pretty far in the future (and leave that vague so that it's far enough out for either of us to not make it one of "those" questions).
As far as what I want to hear... I don't have some "right" answer in mind. The question also helps me understand the person a little better than a typical interview question might. If I know their long-term goals, I can have a better idea if they'll be happy with us and we'll be happy with them.