[nickmain]

The license means nothing unless you're copying Gruber's code.

Gruber doesn't have a trademark on the name "Markdown".

[loup-vaillant]

I don't know about the USA, but here in France, one can have a right to trademark even when it's not officially registered. The main requirement is to show that you have established a de facto brand.

Nevertheless, http://news.ycombinator.com/item?id=4719809