At long, long last, Nintendo has finally gone and put a date to 
Super Mario Galaxy; it's also confirmed some more details on the title's two-player mode and released some tasty new screens and art. November 12th is when American gamers can get their mitts on the super-hyped next installment proper in the Mario series.
Alas, we Europeans don't have a concrete date as yet. What SPOnG can confirm is that we'll see the game by the end of the year, with Nintendo's latest release schedule telling us "Q4" (between October and year's end, for those of you who use real dates).
As for two-player mode in the game, here's what Nintendo tells us in its E3 fact sheet for 
Super Mario Galaxy: "Don’t want to head into space alone? With a second Wii Remote, another player can play alongside by pointing at and collecting bits of stardust, then launching them at enemies. The second player can even sweep aside projectiles or stall enemies to help make Mario’s adventure easier." You can read the factsheet 
here.
SPOnG first 
broke the news that 
Mario Galaxy would feature two-player co-op a year ago, and it was further 
confirmed by Nintendo's website in March, but this is the first time Nintendo has released concrete details on it.
For more on 
Super Mario Galaxy, visit SPOnG's 
dedicated game page.