I would imagine that it will be available at the least because some people may still want to buy it for older Macs, as you suggest.
It also seems like if you're running Snow Leopard then you'd be expected to upgrade to Lion and then Mountain Lion...it's my impression that the Mountain Lion pricing and installation is based on the assumption you're running Lion already. So it would need to be available for anyone who's upgrading from Snow Leopard too, if that's the case.