Here's the license (http://daringfireball.net/projects/downloads/Markdown_1.0.1....) and I don't think Gruber mentioned anything about a "trademark"? It seems like it was mostly about the name of his project just being taken unethically. Just like if someone would take a open source project name and makes it look like an official for when it's not. Maybe not a trademark infringement, but certainly not nice or ethical.