It seems that Namco Bandai has been keeping quiet about a major feature for One Piece Pirate Musou. The usual flying get sources are reporting that this month's issue of V Jump confirms online co-op play for the game.
We don't have official confirmation of co-op play yet, but dummy box art for the game does show online play support for two players.
(If you're interested, the box also lists a hard disk install of 4,400MB, Trophy support, rumble support, and chat support.)
The same sources are also reporting that V Jump confirms Jinbei as a playable character, and that the game is going to be getting lots and lots of costume download content.