In the near future, hybrid customers will be able to take advantage of the REST APIs for both Office 365 and on-premises mailboxes.
The REST APIs (Mail, Calendar, and Contact APIs), simplify programming against Exchange by providing a familiar syntax that is designed with openness (e.g., open standards support JSON, OAUTH, ODATA) and flexibility (e.g., granular, tightly scoped permission to access user data). These APIs allow developers to connect from any platform, whether it be web, PC, or mobile. SDKs exist for.NET, iOS, Android, NodeJS, Ruby, Python, Cordova, and CORS for use in single page JavaScript web apps.
Get more details on the Exchange Team Blog.
