Apple has made an update to OS X 10.8.5 available to developers. Mountain Lion build 12F13 is now available for developers to try out and look for bugs. The new operating system can be downloaded through the Mac Developer Center or through the built-in Mac App Store Software Update functionality.
The build contains no known issues, but Apple has asked that developers continue to focus on Wi-Fi, Graphics, Wake from Sleep, PDF viewing and Accessibility, and Mobile Device Management during their use. This software is just an update to Mountain Lion, with the OS X 10.9 Mavericks developer previews having been seeded to developers earlier this year.
It's not clear when this update will be made available to the general public, though it should be sooner rather than later if there are no known issues with it. We'll let you know when the actual update becomes available for you to download.