Preview at WWDC likely in June, followed by September launch.
Mac OS X Snow Leopard (10.6) Due in 1st Quarter of 2009?
When Apple first previewed Snow Leopard at the Worldwide Developers Conference 2008, they simply stated that Snow Leopard would ship "in about a year" from the announcement. A Q1 release would deliver it earlier than most had expected and makes it conceivable that we could see a demo or announcement at Macworld San Francisco 2009.
Apple has said that they would be focusing on both quality and performance in Snow Leopard. In particular, Apple has made it clear that there will be efforts to improve support for multi-core processors and GPU processing. These improvements will help developers more efficiently use these capabilities that already ship in Macs.