There is also an early API written for Node.js providing profile information and a user's posts as rss/atom/json (also available as npm package): https://github.com/fhemberger/googleplus-scraper

It's still read only, though – haven't seen anything writing back to G+ yet. All approaches to share/crosspost rely either on monitoring your public posts or sharing posts with a special bot user, which is linked to other services like twitter via oauth.