Leviathan comes out in October. There's no description on Amazon yet, but here's what Scott had to say on his blog:
Leviathan is set in an alternate world in which Charles Darwin discovered biotechnology. So the British Empire was built on the backs of strange, fabricated beasties. Living airships! Fighting kraken! Message lizards!(Insert me making Westerfeld-zombie-follower noise here.) Must. read.
PS: I forgot to mention - "Waiting On" Wednesday is hosted by Jill at Breaking the Spine.