Apple sped up production on Leopard to try to preempt Vista but failed to do so. In fact, I remember some features of Tiger (like Spotlight) were made to preempt Vista as well. Now that it'll be ten years before MS produces anything new whatsoever, Apple can take their time with the next OS. Not five years like someone else said, but not one and a half years.
As far as passing up on Leopard, it really depends on what you need. If you don't need the latest builds to stay compatible with some of the newest softare and you don't need the new features of Leopard, then there's no need to update. If it's a cost thing, then there's really no reason to spend the money if you don't want.