There's some built-in games that use the 3DS' StreetPass pass-by communication functionality. One is called Street Pass Quest, and it's a turn-based battle game where you'll work to rescue your Mii. You'll actually use Miis you've acquired via StreetPass to battle baddies to get to deeper stages to get to where your Mii is at. And this is great: If you don't have any Miis, you can use 2 Game Coins to buy a cat in armor to represent you. A guy who got to spend a bit of time with the 3DS says that the first few level's battles were up against ghosts. This all sounds pretty fun for a built-in game.
There's also a puzzle game called Street Pass Puzzle. This same tipster didn't have any StreetPass data to use, so he wasn't able to test it. We do know that the game is a 3D puzzle that is played with pieces collected via StreetPass -- you'll pick up pieces from others that have their StreetPass functionality turned on. The more you interact, the more pieces you get.
Finally, GoNintendo has a few details on the 3DS' power options. There's three "off" modes: Power off turns the system completely off, Sleep keeps StreetPass active, and Suspend is activated when the system is closed or the Home button is pressed.