The "Upgrade" is a bit in the EULA. Your license requires you to have Leopard in order to legally install Snow Leopard. Only the Box Set includes the full license.
Also, apple did the "upgrade only" discs every previous OS launch since panther, I think, with the drop in discs for systems shipped to retail before an OS launch, but sold after. In other words, if you buy a mac today at an apple store, it will probably have 10.5 on the installer discs, with a Snow leopard disc in the box. I do not know for sure if this is actually an upgrade only disc like in the past, or if it's the same full disc you get at retail.