Salesforce Mobile SDK3.1 is an open-source suite of familiar technologies—like a REST API and OAuth 2.0—that you can use to build great mobile apps. Ride the wave of massive mobile device adoption with the new Salesforce.com Mobile SDK, now available to all Force.com and Database.com developers. The Salesforce Mobile SDK supports three development approaches for building mobile apps: native, HTML5, and hybrid.
To begin learning about the Salesforce Mobile SDK, follow this path.
Download the Mobile SDK Development Guide
Here are some other helpful resources for the Salesforce Mobile SDK.
Dreamforce 2014 Conference Sessions
Vendor SDK Ports