The videos we got before release showed us StreetPass causes players' avatars to move into an enclosed area north of your town's shopping center, where models of their houses are located. There they do what zenxacred said above.
I live in the middle of nowhere, so you City Folk (lol) are lucky with all that there StreetPassing. --- Oddworld, Graffiti Kingdom, Supah Nario Bros, and Simtunes FTW! That light at the end of the tunnel might be a train.