As they say, almost every company is always looking to the future. The next version of Windows, despite 8 recently coming out, is probably already in works. Concepts for the PlayStation 5 may already exist at Sony. As it were, the latest rumor out of NeoGAF has Nintendo working on new hardware as of last year. The best guess right now is this hardware is handheld related, but there is always the chance it very well may be the successor to the Wii U. Of note, this rumor is based upon Nintendo already selecting vendors for the product, which they did for the Wii U in 2009 (3 years before it launched). If we go upon the same cycle, you're looking at a 2015 or 2016 launch, which could really line up well for either a new handheld or new console.
If it is a home console, you're likely looking at 2016 to give it a 4 year cycle, but since the 3DS came out first, it may be more likely that a new handheld is in the works.
Almost certainly off-topic, but I don't quite want to make a new thread, because it would go to shit quickly. Also, I'd need to post a source, and I don't really want to get people in trouble. Anyway, Nintendo apparently started working on a new platform in early 2013 and has already selected a vendor for the SoC after talking to several potential candidates. The SoC might be based on an existing design, but will be changed to fit Nintendo's requirements. I assume it's for their next handheld, though.
This leaves us little information is is likely correct given the time periods. The 3DS is due for replacement first, but due it's success they may be choosing to ride it out an extra couple of years compared to a normal cycle and try and use 3DS game sales to help stymie a new home console release. It is also possible that it is a tandem product, new console and handheld coming out together, but Nintendo may not be willing to have that sort of risk with two new products at the same time. Who knows, maybe it is a Nintendo phone? Reality is, the rumor of something being in the works is likely true, but what it is is anyone's guess.