"You do not have to register a trademark to use one or have legal claims (called common law rights)"[1][2]. Sure, registered trademarks provide some additional strength. I said "similarly" for a reason.
Nevertheless, we're talking about who's being a "prick" here, and being a class-A prick has nothing to due with the law. The "Gruber is a prick" vs "Atwood is a prick" debate has been done before: https://news.ycombinator.com/item?id=4716322. If they had stuck with the idea to name it "Rockdown", there would be no problem. In fact, we can now see that Atwood is being dishonest when he says:
> "We then waited two weeks for a response. There was no response, so we assumed that John Gruber was either OK with the project (and its name), or didn't care. So we proceeded."
Gruber had already responded TWO YEARS prior with a "No!"
FWIW, I don't like Gruber. But that's not a good reason to overlook Atwood's dick moves.
Maybe it's time for a little humor? https://news.ycombinator.com/item?id=8272614
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[1] http://inventors.about.com/od/trademarks/a/register.htm
[2] http://www.uspto.gov/faq/trademarks.jsp#_Toc275426681