Snow Leopard is supposed to be the same as Leopard as far as user-visible features are concerned, but faster, safer, more stable. I think Apple is relatively free to choose exactly when Snow Leopard is going to be released. Earlier with less improvements, later with more improvements. They will choose whatever is the best moment in time for Apple.
It will be quite important how the economy goes. The best time to release Snow Leopard from a marketing point of view is when people get back to being optimistic and investing. A company like Apple with plenty of cash can afford to improve its products behind the scene and come out with new things at exactly the right time.