Video of how the Nintendo 3DS might do 3D, based on a DSI game.

The game in the clip above is called 3D Hidden Picture. It's a DSiWare puzzle game, available in Japan, that uses the DSi's camera to track the movement of the console, and adjust the image accordingly to make it appear "3D".

It's entirely feasible - taking into account Nintendo's obsession with keeping the cost of its hardware down - that Hidden Picture is a pointer towards how the 3DS will work.