A new feature has been found in the Mii channel, allowing select DS games to download Miis to the handheld for use in games.
So far, the only game on retail to use this feature is Aruite Wakaru Seikatsu Rhythm DS (or Personal Trainer: Walking), a Japanese title that comes with a free Pedometer.
You can unlock the feature on the Mii Channel right now by pressing A, B, 1, and then holding 2, a new 'connect to DS' button will show, and remain. A video below demonstrates how it works.
Other games to use this feature will be Square-Enix's Wii & DS crossover Crystal Chronicles title, and presumably the Warioware Micro-game creator.