You can now create/edit/delete your own S2 layers:
Once you've made the layers, go back to the normal customize area to select them.
Better docs are coming, but there's a fair bit already up, including the Public Layers browser, where you can see how everything works by example.
Paid accounts only, or it'll work for free on your own LJ installation.
I see zach beat me to the announcement. Feel free to post fun examples of things you're working on.
All questions welcome, however stupid they may seem! (We need to know what people are thinking to lead our documentation....)