While people may disagree with me, I suspect that the 3DS will probably be more powerful than the Gamecube was in order to ensure that Nintendo can port Gamecube games while supporting stereoscopic 3D.
With that said, I don’t see how allowing players to turn off 3D output would translate into developers not supporting it as a feature. Most likely Nintendo will develop the 3DS so that the "driver" for the graphics system will handle the displaying of graphics in 3D as long as they were rendered in 3D. This works out so that most of the games would not see any benefit from not supporting 3D (performance can be identical because it could force rendering both views regardless of whether you are displaying in 3D or not); and the enabling/disabling of the feature can be handled in a uniform way across all games.
