I've always thought Minecraft was an interesting programming challenge (I made a little clone of my own a while back (with portals, for whatever reason): http://www.youtube.com/watch?v=Qk_KjtbHUIs).
On modern graphics hardware, backface culling is done by the hardware itself. It would help if this demo were anywhere close to the limits of the hardware, which it isn't.
riyela is probably referring to the way Minecraft itself draws the world. If you have a solid block floating in free space, Minecraft draws all six faces of that block. However, if you have two solid blocks touching, Minecraft doesn't bother drawing the faces between them. It only draws the faces that are exposed to air (and are, therefore, visible), so it doesn't need to draw nearly as many faces.
That should get a decent performance improvement.