These details come from Nintendo Power...

- 16 players online confirmed
- worldwide online play
- region-specific play as well
- use friend codes to play online with specific people
- Online game modes: Deathmatch, Team Deathmatch, Capture the Flag
- Medical Facility, Training Complex are some of the multiplayer maps
- All multiplayer maps taken from single-player
- WiiSpeak support reconfirmed

You can learn more details about the game in general here