Though the update comes with no known issues or new features, Apple has asked developers to focus on AirPlay, AirPort, Game Center, Graphics Drivers, and Safari in their testing.
This is the third beta release since the first 10.8.3 beta was seeded back in November. Registered developers can download it via the Apple Developer Page.
Update: 9to5Mac reports that Apple has now released build 12D44 to developers, some two days after 12D43 was released. This release still reports no known issues, though it's possible the quick update is a fix for some unmentioned bug.