One commenter today said... and "the Sold Out Countdown Begins", and yes, Shield of Baal Leviathan is already sold out. It was gone in a matter of hours, and now to get one, it will be a mad rush to your local game store to get one when it comes out.
Ded'ard posted a PDF of the free preview for Leviathan that was available via the Warhammer app on iOS
You can also see the free previews at this link, and a couple very intesting ones below.
There are lots of army bundles up as well today for pre-orders as Games Workshop lays it on thick as we head into the holiday season.