Well, I don't think the original poster's idea has much legal merit, but I think a few people misunderstood what he was saying (or maybe I'm musunderstanding it).

I don't think he was saying the 18th ammendment applied to weed or that the 21st made weed legal by "analogy." I think he was saying that it required a constitutional ammendment to authorize the laws to make liquor illegal, so it should require a similar constitutional amendment to make weed illegal. He's saying federal laws prohibiting marijuana are not constitutional without an ammendment to authorize them, just as the Volstead Act prohibiting alcohol apparently would not have been legal without the 18th amendment to authorize it.

Anyway, that's how I understood it.