They didn't expect to be able to keep it a secret. They were worried it would leak. But the co-presidents of Naughty Dog managed to keep the fact that their Uncharted studio has been making The Last of Us for a while. Moments after the first trailer was revealed, the two chatted with the official PlayStation Blog and teased that the short trailer contains "lots of clues and little teases in there about what the game is about."
So scour it some more. We've rounded up some details about The Last of Us, but surely there are things we've missed.
Naughty Dog's First Comments on The Last of Us [PlayStation blog]