Reggie announced this news. I thought that the DS doesn't have an internal memory enough to store a game. I guess they will have to sell a blank card.
Anyway, I think this will be great for people who have both DS and the Wii. I personally am looking forward to it.
"Speaking with the New York Times, Nintendo's Reggie Fils-Aime has offered a little more info on the Wii's ability to interact with the DS. While we already know that the Wii will be able to download and transmit DS demos to the handheld, we now also know that you'll soon be able to download full games to your DS."
http://kotaku.com/339362/reggie-says-wii-can-download-complete-ds-games