Last I heard, Nintendo put it on indefinite hold, which means it's probably cancelled. I guess there's a chance that they delayed it for this year due to Mario Baseball, but I wouldn't hold my breath.
One other possibility is that they decided to prepare it for Revolution. The GameCube already has no reputation for sports games (outside of the Mario sports) so this game would have a much bigger impact on a new system than on the current one. Still, I wouldn't hold my breath.