Sega sent Famitsu a few details about Sonic Generations. The magazine confirmed Metal Sonic from Sonic CD is in the upcoming game as a rival for Modern and Classic Sonic. Metal Sonic is said to have a classic style. I suppose that means Metal Sonic’s model is based on his Sonic CD sprite.
Sonic Generations has Chemical Plant Zone from Sonic the Hedgehog 2 in the game. Like all stages, there will be classic style and modern style versions of this level.
On Nintendo 3DS, Sonic Generations has an original special stage where Sonic runs into a tube in stereoscopic 3D.
Sonic Generations will be available for PlayStation 3, Xbox 360, and Nintendo 3DS with different content later this yaer.
Update: Here’s how Metal Sonic looks like Sonic Generations.