This already exists. A lot of WordPress blogs use it (I think WordPress might have acquired the company that created it). I have disabled it on my WordPress(dotcom) blogs because it's just a PITA. It takes too long for a preview to be created and it blocks the blog -- which is an issue if you accidentally run your mouse over it (like those stupid popup Share buttons all over the place now).

EDIT: I've done a screensnap to show you what it looks like in my WordPress Dashboard for Comments (where I apparently cannot turn it off): http://mikecanepics.wordpress.com/2013/01/05/site-preview-in...