Sonic Generations was just confirmed for 3DS through the latest Nintendo Power, and now we have more details on the nature of the gameplay. Chief of 'em all is that both play styles -- Classic and Modern -- will be side-scolling. The former will consist of the momentum-based action of the Genesis originals, while the latter will follow in the footsteps of the Sonic Rush series.
Aside from Green Hill Zone, no levels will be shared with the game's console brethren. Also confirmed is that Casino Night Zone and Classic Eggman -- I'm sorry, Classic Robotnik -- will appear. Yay!
The only niggling tidbit that might worry you is that the game is being directed by Tetsu Katano, the man who directed Sonic and the Black Knight. Let's pray that's not an ill omen.
Nintendo Power Sonic Generations 3DS Details [The Sonic Stadium]