There was an interesting thread on this subject a few months ago.

http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/59150/1.html

My impression is that boarding schools select for parents' socioeconomic status, not LOG of students. Of course, there are plenty of very gifted kids at those schools, but plenty of kids who aren't. So, going to an expensive boarding school is no guarantee that your kid will be surrounded by very gifted kids.