Prepare to clean another fridge.
Radio Free Nintendo has predicted it for a few years, and now it's finally come to pass through My Nintendo: DS games will be available to download on 3DS.
The first applicable title is WarioWare Touched, which is in the initial batch of My Nintendo rewards at a cost of 1000 Platinum points. The DS WarioWare launched in North America in 2005, and was one of the first DS titles to be downloadable on Wii U.
The games will run as DSiWare, meaning they are stored on the 3DS's internal memory as opposed to the microSD card, and will have to be managed separately - WarioWare Touched is 214 blocks. The only previous downloadable DS title was a Japan-only release of Advance Wars: Days of Ruin as a Club Nintendo annual reward.