Joystiq to Charles Martinet (the voice of Mario and many other Nintendo characters) can coyly hint at things of interest to the world. Why just yesterday morning, Martinet dropped hints at an "Awesome new Mario game." Normally such a statement would provide us little in the way of excitement, except it came paired with "And it's not NSMB [New Super Mario Bros.] Wii, and it's not Galaxy 2," as well as assuring us "it's going to be crazy fun!"
Considering the recent release of Mario and Luigi: Bowser's Inside Story and the two he mentioned in his Tweet, we're fairly sure Mr. "It's a me, Mario!" is dropping his dulcet tones into an unannounced "new Mario game." We've shot Nintendo a note for some clarification and will update this post as soon as we know more.
Update: Unfortunately, The Big N hasn't offered us much in the way of information, saying, "We're always working on new games and experiences, but we have nothing to announce at this time."