I went to the US Patent and Trademark Office (USPTO) site and searched for the word "hackathon" in their (US) trademark database and it didn't find anything, so US-based hackathons should be fine (for now).

In Germany, I'm sure the Germans can come up with some long-German-word-like-'marathon-of-hacking'-that-means-the-same-thing-as-hackathon, if they haven't already.