Nintendo has registered the domain 'supermario4.com', possibly outing the platform holder's next Mario title. The domain is owned by Nintendo of America and currently redirects to Nintendo's official website.
In an investor meeting in January, Nintendo president Satoru Iwata said that the company was working on a 2D Mario game for 3DS, with a planned release date by the end of the this fiscal year. It's currently unknown if this registration and that title are related, or for wishful thinkers if it's a Mario launch title for Wii U later this year.
We'll probably find out more at this year's E3.