Console wise, they need to avoid having to render the scene twice (once for each eye). Halving the resolution helps pixel side, but doesn't help vertex side since all verts would still have to be processed twice. I'm guessing they could implement 3d with some sort of mrt approach, where you turn on a "3d mode" render state, and the hardware would automatically enable two color output buffers, and use the resultant z values to automatically shift the final pixels accordingly, so each shader would write out the same color to two different buffers at two different locations. That way vertex load stays the same in 2d or 3d mode, and they could render 3d at full res since pixel load would also be the same (all shaders are run only once). It should be easy to toggle 3d on/off that way. The only hit is some bandwidth.
This method should be doable on existing consoles as well, although there is little incentive to support 3d games anytime soon.