After only four months, the Nintendo DSi has reached an installed base of 2 million units in the U.S. faster than any system for which Gamasutra has data.
The PlayStation 2 reached 2 million units in March 2001, five months after its October 2000 launch. Microsoft's original Xbox didn't reach the 2 million mark until its sixth month while its successor, the Xbox 360, took seven months, mostly due to supply issues in early 2006.
Nintendo's Game Boy Advance and GameCube each took four and a half and eight months, respectively, to achieve an installed base of two million units. The original Nintendo DS took nine months, while its successor, the Nintendo DS Lite, took between four and five.
With that said, the Nintendo DSi also faces a very strong competitor which has slowly claimed market share over the past three months: the Nintendo DS Lite.