Nintendo's press release [PDF] wasn't sent to news outlets, but it's sitting undisturbed at Nintendo's website. It says the handheld console will launch in the fiscal year that ends in March 2011 (so late 2010 or early 2011), and that it will be fully unveiled at the E3 gaming expo in Los Angeles this June.
The 3DS will will add some motion sensing technology to its control scheme, and it will play your old DS and DSi games. Nintendo doesn't say whether it will keep the two-screens-and-a-stylus layout that we've all become accustomed to on the existing handhelds.
Nintendo's Wii and DS consoles have turned the video game world upside down, using unique control schemes and affordable hardware to attract millions of people who were never before interested in video games. 3D is already a big deal, and Nintendo hopes that whatever 3D tech it's using will attract even more people to the video game hobby.
Note that Sony's PlayStation 3 will go 3D on supported TVs this year, though, so Nintendo's neither alone nor first.
(The picture included here is not of the 3DS but the already-out DSi. No photos or renders of the 3DS have been released yet.)