It's single player only unless you go online according to the box. Epic failure on Sony's part.
You can do 1 - 2 player co-op split screen in story mode right out of the box. Once you beat the tracks in story mode it unlocks them for more players.
I played this last night and did 1 - 2 co-op. I didn't verify doing more, but that's what I read in a review.
Very strange way to do co-op on a karting game.
Edited by omyard, 24 November 2012 - 04:49 PM.