has just announced the release of OS X Developer Preview 4.
Take advantage of new features in OS X Mountain Lion, likeCenter, Notification Center, Documents in the Cloud, advanced security features, and Facebook integration, and start preparing your apps for the Retina display. Visit the Dev Center to get the OS X Mountain Lion Developer Preview 4.
Developers can click here to download the new version of OS X Mountain Lion.
Note: There also appears to be a separate OS X Facebook Developer Preview that can be downloaded directly from the Mac Dev Center.