I was lucky enough to score a signed copy of Leviathan at BEA back in May. Bill immediately seized it so that he and his son (8) could read it together, but I've finally got it back and my daughter (7) and I are really enjoying it. (Yes, we know our kids aren't technically young adults yet, but the book is that good.)
If you haven't heard about Scott Westerfeld's amazing steampunk novel yet, if you haven't picked it up, don't take our word for it, watch Scott's pitch: