Deus Ex: Human Revolution surprised everybody when it was released by actually being a good prequel to a ten year old game. The game did respectable sales as well, so the common thinking is that a sequel to Deus Ex: Human Revolution would likely be in development. Yesterday we may have gotten our first piece of evidence that the sequel is development as Square Enix officially trademarked Deus Ex: Human Defiance.
However, there are other possibilities besides the new trademark being the first clue to a sequel. First it is known that Straight Right is porting over a major Square Enix title to the Wii U, so this trademark could have something to do with a Deus Ex: Human Revolution Wii U port. The other thing the trademark could be related to is the Deus Ex movie that is currently in the works.
According to the Deus Ex: Human Defiance trademark filing, it covers both games and feature films, so it could go either way. However, with the success of Deus Ex: Human Revolution it would probably be more likely that this new trademark does have something to do with a sequel. But Square Enix has refused to comment on the issue so we will just have to wait to E3 to probably hear something official either way.