Multiplayer
During an interview with IGN,[4] Takashi Tezuka, Nintendo's analysis and development's general manager, stated that multiplayer for Super Mario Galaxy is more likely to be co-operative. Two-player functionality was already confirmed for the game. The development team is already experimenting with new ways to use the Wii Remote so that one player can control Mario while the other aids him. Conversely, Miyamoto also suggested that the second player could have the ability to interfere with Mario's progress.
from wikipedia...
and I read also that they could posibily be a 4 player mode where the 3 other player change the world around mario (player 1)