As several people have spotted, and as has been reported on sites like Destructoid, the trailer for Watch Dogs that we saw during the PS4 announcement includes a URL that you'll only spot if you're paying really close attention and which leads to what appears to be a teaser site for the game. You can see the URL on posters throughout the trailer, for instance at 49 seconds in.
The URL is supposedly for the in-game company ctOS, which is presumably the Watch Dogs version of Google or Apple, but in the real world it leads to a black webpage with white text and ASCII imagery.
The site is headlined “DEDSEC” (spelled in ASCII), with the subheading “Dare to Resist”. At the bottom it reads, “This domain has been seized by DedSec”. In the source code, we learn that the title of the page is “Own3d by DedSec”, and we get another ASCII image of “DEDSEC” with what appears to be a snake around it.
But the interesting part is the statement in the centre of the page, which is in Latin and reads: “Quis Custodiet Costodes Ipsos”. According to Wikipedia, this phrase is generally attributed to a Roman poet called Juvenal and means “Who will guard the guard themselves?” or, more poetically, “Who watches the watchmen?” This is apparently how the comic book series Watchmen got its name. So it's pretty clear what kind of a message Ubisoft is trying to push with this game. Does it interest you?