Twitter has filed an application with the United States Patent and Trademark Office (USPTO) to trademark "subtweet." A subtweet, for the mercifully uninformed, is a passive-aggressive tweet that mentions someone without using their Twitter name. Before anyone goes around screaming about how this means Twitter now owns the word, that's not how trademark works.
The application was filed on October 30, and opened up for opposition on November 19. Left unopposed, Twitter will likely get the trademark in a few weeks.
Trademark is a recognisable name, symbol, sign, etc. which distinguishes a product as coming from a particular source. It's a commercial mark, meaning that Twitter's control of it would exclusively be in commercial settings. So no one can name their new product "Subtweet" or sell things that say "Subtweet" on them.
The grounds for going after someone for trademark infringement are "likelihood of confusion", which is basically how likely it is that a consumer would think that Twitter was either a source or a sponsor of the thing being offered. Saying something is a "subtweet" isn't likely to fall under that category, although selling a shirt that says "Subtweet" is. The reason people think that trademarking something is a big deal is that not enforcing it can lead to the mark growing weak, making it harder to enforce in courts.
I'm betting it's that latter issue that led the Twitter filing a trademark application. A bet backed up by Twitter's statement to the Verge: "When you need to protect your namespace from people who would misuse it. #keepingit." The other reason? Twitter lost the battle to trademark "tweet" after app developers used it for ages without Twitter registering for a trademark. Twitter's application for what they intend to do with "subtweet" is basically identical for the one they had for "tweet". There isn't the same developer issue with this one, which is probably why it's made it further in the USPTO process than "tweet" did.
Twitter is going to have to use "subtweet" as a trademark on products in a genuine — i.e., not just to keep the trademark — way. Trademark is a "use it or lose it" deal. If you don't use the trademark, it goes away. Twitter's probably learned from its last trademark battle to leap whenever they think they can.