It's been known for quite some time that Nintendo was working on a 2D Mario Bros. game, though we assumed it would likely be a follow-up to the New Super Mario Bros. franchise, either on the 3DS or the upcoming Wii U. The NSMB games have sold absurdly well, after all, so it would make sense for Nintendo to continue growing that franchise.
New news, however, has many believing that the next 2D Mario game won't be New Super Mario Bros 2., but Super Mario Bros. 4.
Nintendo has reportedly picked up the domain name "supermario4.com," hinting that the publisher might be pulling a Sonic the Hedgehog 4 and continuing to grow the platformer series that started it all. While this is, obviously, pure speculation, it sure makes sense, though the brashness of Nintendo to purchase the domain name without attempting to hide it (Nintendo of America is listed as the holder) is somewhat strange.
There's always a chance that this is just Nintendo scooping up a bunch of domain names "just in case," though we're going to choose to be sensationalist and excited regardless. The prospect of a true 2D Super Mario Bros. 4 is simply too awesome for us to let anything get in the way of our enthusiasm. Sorry.