Sonic Generations is getting a 3DS version alongside its console counterparts, and today Sega has detailed how that version will use the system's SpotPass features. You can exchange profile cards and exchange missions, but missions can also be unlocked using Game Coins.
Andriasang details the features. First, the game includes 100 missions outside the main story, which can be unlocked via SpotPass. Exchanging a pass with another player lets you receive a mission that they've cleared, so you're basically infecting people you meet with more Sonic. These stages might involve taking out a number of enemies or going through a regular stage modified with different enemies.
If you'd rather walk your way to extra Sonic content, you can unlock the missions with Game Coins obtained from putting steps on your pedometer.
The profile cards are a little record of your Sonic fandom, letting you fill out fields like how long you've been playing the Sonic series, your favorite Sonic character, your records in Sonic Generations, and your views on furry cosplay. Okay, I made that last one up. At any rate, exchanging SpotPass transfers will update your card.
[Screenshot from console version of Sonic Generations.]