One of the more useful ways the Nintendo 3DS will be able to interact with Miis is its ability to take a person's picture then automatically generate an avatar based on the results. Here's that feature in action.
As you can see, all you need to do is take a photo of your (or someone else's) face, and after a little crunching (during which you get that cute little animation sequence) the 3DS spits out a Mii based on your facial features.
Or what it thinks are your facial features; I'm sure patience and a mirror could have achieved better results on a Wii than the 3DS managed in this instance. Then again, this will be mighty useful for those times you just want to make a Mii based on a friend and can't be bothered with the 5-15 minutes required to do it by hand.