Technically you need Leopard, but it will be interesting to see how or if this is enforced.
Rather than buying Leopard now though, I would wait and see what your options are once we have more information, i.e. buying an updated Mac Pack (Snow Leopard, iWork '09, iLife '09) would probably be cheaper or the same price as upgrading to Leopard now then Snow Leopard later in the year.