Since September 2013 Silverstripe 3.1 is available. I could test that version already for some weeks and I based also the silverstripe portfolio on Silverstripe 3.1. I can say it runs stable and the new features are very helpful for Content Editors, especially if they have to content-manage a Responsive Website.
With the new "split view" you can preview your changes in Draft and Published Version. Additionally you can preview your website in common screenwidths like "desktop", "tablet" and "smartphone". Furthermore the Publish and Save as Draft Buttons (etc) are now more easy to use (minor actions were moved into a "more options" panel).
Here you can see the complete Release Notes.