During the latest Iwata Asks Nintendo president Satoru Iwata revealed that Super Mario 3D Land will have a time attack mode once the main levels are completed. With StreetPass enabled you will be able to see the times of people you have passed and determine whose skill level is similar to yours.
“To be honest, it was so hard that at one point we gave up, but Koizumi-san said he definitely wanted to put it in. (laughs) After you clear the normal stages, you unlock the Time Attack mode and can see the times of people you have passed and determine whose skill level is like yours. You have a friendly race with people about your level and enjoy shaving away your times.”
“This time, you can even pass by people who don’t have the game! If you pass by someone’s Mii character someplace like the StreetPass Mii Plaza application, you can receive an item from that Mii character at Toad’s house. If you pass by the same Mii character again, you get something better.”