Nintendo's position and the law are two completly different things. Nintendo doesn't want you to back up your games because they're afraid you'll use them to pirate.

However, the law states that as long as you own a game, you're allowed to make one, and only one, backup copy of your own game (as referenced in these links, but I guess you'll have to read them at home). If you sell your game, your backup copy has to be either destroyed or packaged with your original. They must always be together.

Nintendo disagrees with this law, but unfortunately for them, it's still the law. You may back up your Nintendo games. They don't like it, but you can still do it.

EDIT: Another post got in there. Let me readdress the 24-hour law. There is none. That's something that emulation sites put on their site to give the illusion that what they're doing is legal. I'm not going to relink my links, but if you go up there, it's the third from the top of my four links that talks about the nonexistance of this law.