Just a few moments ago at their annual E3 2012 press conference, Nintendo announced that they will be bringing a new game in the Paper Mario saga to the 3DS. It's called Paper Mario: Sticker Star and it features much the same gameplay and style as older games in the series.
The big difference this time around (according to the reveal) is the new "sticker" system. Mario will now be able to find these "stickers" around the world, pasted to the environment, and they will unlock certain abilities and secret areas. For instance, one sticker may give Mario a "stomp" move in battle, while another may reveal a hidden location.
They didn't reveal much information about the game beyond that (the 3DS wasn't given very much time on stage) however, they did mention that the game will be releasing in 2012. However, they didn't get more specific than "holiday 2012, though that presumably means November or December.
We'll have more on Paper Mario: Sticker Star when we get our hands on it on the show floor.