There was a heavily updated Cave Story for 3DS that released last year, but this isn't that version. This is the original 2D version of Cave Story updated with some 3D visuals.
This is basically the Cave Story+ that is available on PC through Steam, but without the HD art and music. It will have layered 3D effects taking advantage of that little switch to the right of the top screen on the Nintendo 3DS, though.
The game will be available on October 4 for $ 9.99.