You're in the wrong thread. This one has nothing to do with previews.
Actually, Brett opened up that possibility early on in the thread, in his third post
Imagine, just mousing over a result on a serp, and having that result open up further and further - showing you more-n-more information from that site. You would never have to leave Google again. That is the kind of ajax testing Google is doing.
But no one has currently reported that kind of thing actually showing up, so for now it's just an AJAX possibility that's been raised.