A poster on the Sonic Retro fansite came across Sonic Extreme, an unreleased project by Sonic Team for the original Xbox. The skateboarding game looks to be similar to the skateboarding section from Sonic Adventure 2, which was so damn pointless as Sonic can run really fast. Like really really really fast.
Anyway, the poster on Sonic Retro says Sonic Extreme was based on the Sonic Adventure 2 engine and featureed a split-screen mode with Sonic and Shadow as playable characters. Obviously the game was never released and if anything, probably transformed into Sonic Riders for the Xbox 360 Kinect.