In the latest volume of Iwata Asks: Super Mario 25th Anniversary, Shigeru Miyamoto mentions that he is "making a new Super Mario Bros. game for the Nintendo 3DS system." Though this doesn't exactly come as a surprise, it's nice to have confirmation. No other details were given, but I'd say it's likely that we'll hear more about the game during this year's E3.

Link: Iwata Asks: Super Mario 25th Anniversary Volume 5