According to this IGN piece, Level-5's White Knight Chronicles is to feature online co-op. From the story:
White Knight will feature a cooperative online quest mode. Separate from the single player story mode, the online mode, known in the Japanese version as "Live Part," will allow four players to team up to work through quests.
While the game's story mode does not offer co-op play, there is a connection between the story mode and online modes. The character that you use in the online mode will travel in your party during the story mode. Additionally, quests from the online mode can be played single player as well.
White Knight will feature all the expected components of an online title, including a lobby, featuring up to 16 players, an auto matching system, and chat facilities of some form. A robust character creation system was detailed months ago.
The game's out in Japan on Christmas Day, and will be playable at TGS this week.