In an article about the Louvre's 3DS integration, Spanish newspaper El Mundo said that Miyamoto revealed plans to show a new Super Mario game for Wii U at E3. However, without a direct quote, it seemed likely that he had been talking about the Wii U and the (confirmed) 3DS Super Mario game, and the two had become conflated. The other, less likely option, is that Miyamoto had let slip the existence of an unannounced game, which is out of character for anyone at Nintendo.
However, that's exactly what happened. Eurogamer received confirmation from Nintendo. "In a recent interview, Mr. Miyamoto confirmed that a new Super Mario Bros. game for the Wii U system will be shown at this year's E3 Expo," the Nintendo representative told the site. "We'll have more to announce about our plans for the E3 Expo at a later date."
During the last E3, Nintendo showed a Wii U game (above) based on New Super Mario Bros. Wii. It's possible that demo may have evolved into this new game, or it could be something totally different. All we know at this point is that it's a "Super Mario Bros. game."