From what I remember, Steve never explicitly stated for Panther or Tiger that there were "Top Secret" features they weren't showing. I remember for Tiger being slightly disappointed that there turned out to be nothing much new that wasn't shown in the preview, that clearly isn't the case with Leopard.

It's probably true that part of the "Top Secret" has to do with it not being fully done, but I also think there is some reason to think MS could copy something from the new Finder, especially if we are just talking look and feel.