He may have selected the particular syntactic conventions it uses, but the idea of parsing ASCII markup and converting it to a formatter's input syntax is much older. I'm aware of prior art dating back to 1986 or so -- and wouldn't be surprised if that wasn't the first either.
Right, so, for the sake of argument, you support perpetual copyright and software patents? Just trying to gauge how much exclusive ownership you think people should have over ideas.
Some people want to make a Markdown spin-off: clean, standardized, tested etc. Nothing ever stopped them, but they could really use the prestige given by the original name.
The question is, does John Gruber have the right to that name? I think he should give it away, but if he won't, we probably shouldn't force him. I'd change my mind if someone makes a compelling case against trademarks.