This is an article I remember seeing a while back, and then recently bubbled up again when someone else posted it into their Friendfeed stream. It's a pretty good article that I enjoyed reading the first time, and again the second time it popped up.
How can I let the introvert in my life know that I support him and respect his choice? First, recognize that it's not a choice. It's not a lifestyle. It's an orientation.
Second, when you see an introvert lost in thought, don't say "What's the matter?" or "Are you all right?"
Third, don't say anything else, either.