I do see there are many sites use trademark. like androidforums .com or iphoneforums .net, iphone and android are trademarks. You can find many such examples.
I feel it is bad idea to use trademark but that is my view.
Here is what Jeff says…
https://meta.discourse.org/t/is-discourse-trademarked/8681/2?u=sat
again, I’m not a lawyer but there may be some way…that is why Communiteq (formerly DiscourseHosting) is using discourse in domain name.