I've read in many places that it would probably come out at Macworld.
No one knows exactly when Snow Leopard will come out. The main reason why people are saying it won't come out until Summer of 2009 or later is because Steve Jobs said at WWDC 2008 that Snow Leopard will come out in about a year. That would then make Summer of 2009 "about a year" from when he announced it would "come out in about a year".
MacRumors link