I think another study that would be worthwhile doing was to look at kids that are specifically WAY ahead in milestones (not just slightly advanced but years advanced). So look at the kids reading at 2 or speaking 100 words at age 1. I'd be pretty surprised if they don't find a high correlation then!

One obvious thing that is worth repeating is that these studies have to be pretty culturally specific. For instance, I've seem some developmental trackers that have said my daughter should be rolling a ball back and forth at a certain age. We've never actually shown her that because balls are meant to be kicked (we're soccer fans if you couldn't guess it :D) so that would also skew results. FWIW, we did show her awhile back and she started doing it that very day...