Two links from IGN-DS.
The menu system.Sleep mode.Apparently when you turn on the system, you'll see an icon-based menu on the touchscreen, asking if you want to launch whatever DS game is in the DS slot, or a GBA game from the GBA slot, or download a single-cart multiplayer game from somebody within range, or otherwise connect to people, like for Pictochat or something.
"Sleep mode" is going to be built into the system itself, and is apparently going to be a part of every single game, so you can automatically "pause" your game and put it into sleep mode, just by closing your DS and putting it into your pocket.