A DS homebrew developer by the name of smealum is taking indie smash Minecraft and bringing it to Nintendo's portable system. Unofficially, of course.
At this stage there's not much running but the world itself and the ability to smash blocks around, but it's a start! Hopefully he can get some other stuff, like random world generation and crafting, up and running soon.
Those willing/able to run homebrew on their DS, you can find a link to download the above tech demo here.