Though two dimensional like the old games, it will still display in 3D

At an investor meeting in Japan today, Nintendo dropped little bits of information about the 3DS and Wii U. While no logo was shown or name given, Iwata announced a "totally brand new" side-scrolling action Super Mario 2D title. Thats 2D in the vein of New Super Mario Bros on the Wii and DS, as it will feature stereo-3D like all 3D games.

The game will be out within the next financial year for Nintendo, so some time after June (most likely for Christmas). Expect to see more at E3!

Source: Nintendo