The survival horror title is apparently being delayed by what Naughty Dog claims is its need to set a very high quality bar for every aspect of the game; and the developer wants to make sure that the title raises the bar and lives up to the very high internal standards of the developer.
With the delay, this will see The Last of Us ship the week of E3, and it is hard to say what the expected impact will be on all of the other Sony announcements of the PS4 and the titles to go with it. Both Sony and Naughty Dog say that the decision to delay the title was a difficult one, but that it was better than compromising the original vision they had for the game.