Apple has made a pre-release of OS X Mountain Lion 10.8.3 (Build 12D32) available for download on the Mac Dev Center today.
According to an email received by TUAW, the seed note mentions that focus areas of the update are AirPlay, AirPort, Game Center and Graphics Drivers. One known issue -- Safari 6.0.2 won't be available to 10.8.3, so it's recommended to update to the latest version of Safari before installing 10.8.3.
There are some other odd quirks, including after waking from sleep, when some background graphics may appear in black or become transparent. In addition, when using Command+Tab to switch between apps, icons may appear in black.
Thanks to RL for the tip!
Apple OS X Mountain Lion