It's REST/JSON. Right now, you can only get a list of all publications, and any specific publication. Publications include both posts and comments. Only public data is available—you can't get mail, deleted, or drafts, even your own. Because there's no login mechanism.
It's also a little rough around the edges. For example, you'll see keys for search_text, search_title, and search_url. Those are used internally. They'll be removed from the external API at some point.
This obviously isn't enough to make anything interesting, like a mobile app. But you can play with it.
For the immediate future, I'll probably be alternating between adding to the API and making the main app more scalable.
No promises, but feel free to make requests.
EDIT: added an endpoint to get all children of a publication recursively in a single request:
EDIT: changed API from hubski.com/api to api.hubski.com.