Nintendo announced in a press release this morning that it managed to sell more than 12 million hardware units in the United States during 2011. That figure is comprised of DS, Wii and 3DS hardware. The Wii sold 4.5 million units in 2011, the DS sold 3.4 million, and the 3DS sold 4 million, according to Nintendo.
The 3DS managed to sell more units than the Wii did in its first nine months of availability, according to the company. Of course the 3DS enjoyed something that the Wii didn't need in its first year of release - a major price cut.