The crew mentioned it on the bombcast a few times. I think specifically around PAX East. All about swapping different characters or items over SpotPass to defeat enemies in some kind of labyrinth... It sounded like it was possibly a nintendo built-in. After dusting off my 3DS, and finally quitting out of Zelda: Phantom Hourglass to do system updates, I didn't see anything that looked like what they were describing.
Does anyone know which app or game they were talking about?