WP8.1: Dynamic lock screen designs NOT included in dev preview
At Build they demoed 3rd party dynamic lock screen layouts which will be available as apps in the Store. They said they're working with developer Rudy Huyn on a set of designs.
Joe Belfiore just tweeted that those will not be included in the developer preview, and that they don't have a release date yet.
Dev preview will NOT include the lock screen theming app. That will come later. I DO NOT KNOW the date. :)
I don't think they have released the API for this yet, so don't expect any dynamic lock screens to be available anytime soon.