Game Spot has this to say:
GameSpot can now confirm that Sonic Team’s secret GameCube game is indeed a port of Sonic Adventure 2.
Details regarding GameCube-specific features have not been revealed at this time,
but the game is listed on the official Space World 2001 games
lineup. GameSpot will have more on the GameCube version of
Sonic Adventure 2 later this week, direct from Space World.
Go here for the article
http://gamespot.com/gamespot/stories/news/0,10870,2806025,00.html