Preview at WWDC likely in June, followed by September launch.
Apple Seeds First Mac OS X 10.6.8 Build to Developers
No details on changes included in the update have yet been revealed.
Work on Mac OS X 10.6.8 comes as Apple continues to move toward a "summer" release of Mac OS X Lion, which has been in developer previews for some time now and will undoubtedly be a major focus of Apple's Worldwide Developers Conference early next month. Consequently, Mac OS X 10.6.8 may be the final maintenance update for Snow Leopard, depending on how long it takes to make it through the refinement process and Apple's final timeframe for launching Lion.