ARDefender is an upcoming game for the DSi. It uses the camara and a piece of paper (with a design) you print off, point the camara at the paper, and PRESTO! A tower appears. This is Augmented Reality, mixing the virtual world with the real world in realtime. The 3DS will have ARcards, this is like that.

In ARDefender, you have a tower. You move the camara around to aim and shoot nasties that are attacking. The goal is to survive as long as you can. There are 5 diferent enemies (on iPod) and 3 different weapons, machine gun, rockets, and laser (on iPod). I have played this game, on iPod, and it is incredibly fun. There is very little lag when you move around. For a video and screenshots (iPod gameplay) go to ARDefender.com !
On that page, it says COMING SOON for DSi. I hope it is soon.