In addition to a notifications system that spans across WordPress.com and WordPress sites with Jetpack installed; the new Jetpack also has a great JSON API.
Originally posted on Jetpack for WordPress:
Jetpack 1.9 is here. That’s right, it’s time for another big helping of Jetpack awesomeness. This release brings you Toolbar Notifications, Mobile Push Notifications, Custom CSS for mobile themes, a JSON API, and improvements to the Contact Form.
Notifications adds a menu to your toolbar that lets you read, moderate, reply to comments from any page on your blog. Plus, if find yourself on TechCrunch, GigaOm, or any of the millions of other sites running on WordPress.com, you’ll be able to view and moderate comments on your own site from the toolbar there, too.