Naughty Dog detailed the pre-order bonuses for The Last of Us on the official Playstation blog, and in doing so also revealed the existance of multiplayer in the game. Skins, more in-game currency, and "multiplayer melee attack booster" certainly seem to suggest that.
"Multiplayer, wha? We're not quite ready to talk about it yet, but stay tuned as we'll be revealing more details on multiplayer before The Last of Us is released on May 7th, 2013," Arne Meyer, community strategist at Naughty Dog commented.