"One of the great things about the multiplayer and the co-op is that it gives us great opportunities to expand the experience for players with downloadable content, unlockables, things like that. Which of course you don't really do with the single-player game that much. We tried to pack a lot of value into the first game in terms of unlockables, but with that online component the game can kind of keep growing, especially based on players' feedback too."
Link - http://www.psxextreme.com/ps3-news/5526.html
Good to see that the game is going to remain fresh with DLC, mainly for the Multiplayer feature of the game!